I am the kind of player who likes to figure out the "optimal" strategy, in TTRPGs, in board games, in video games, etc. Not only that, the character I play (an abjuration wizard) shares this trait with me. So I will answer from the perspective of a player trying to avoid this issue for myself.
Play the character, not the game
The main strategy I've adopted to avoid decision paralysis in combat is to keep the focus on role-playing even during combat. By role playing, I don't mean continue spouting witty in-character dialogue. What I mean is, on my turn my objective isn't to make the "optimal" move, as it might be in the case of a board game. Instead, my objective is to decide what my character would do in the current situation. To do this, each round I ask myself first "What does my character want to accomplish right now?" and then "What is the most obvious/direct way to do that?".
(As some comments have pointed out, this answer is effectively a frame challenge, because my advice for the player who "wants to make optimal decisions" is to abandon that goal entirely in favor of a new goal, in the hope that the new goal will be both more manageable for them and more fun and rewarding at the same time.)
To demonstrate this principle, I'll give some examples from my past sessions. As the player of an abjuration wizard, I have a similarly dizzying array of combat options to choose from to the player in question. By just keeping in mind the one or two most important things my character wants to accomplish at any given moment, it makes my decision a lot easier. For example, in one combat, we were facing off against a bunch of soldiers and one powerful spellcaster. As an abjurer, my primary objective in that fight was to keep the spellcaster in check. This meant always maintaining a position to ensure I was in line of sight and within counterspell range, and using my action mainly for dispel magic to shut down any of his spells that made it through my counterspells. Once the enemy mage cast globe of invulnerability, this meant running up into melee with the mage to get inside the globe, so I could continue targeting him. Later in the fight, the mage cast banishment on my character's romance interest, and my primary objective shifted from control to dealing maximum damage to the mage in order to break his concentration on the banishment spell.
I'll give another example where my goal was completely different. In another battle, I had just punched a hole in a wall with my Arcane Hand to let the party into a warehouse where our assassination target was hiding. As the rest of party made their way in, a dangerous high level enemy came running toward the warehouse to protect the target. Because we had a good idea of what enemies were inside the warehouse, I judged that the rest of the party could handle them without me. So instead of following them in, I made it my mission to keep this one high-level enemy out of the fight long enough for the party to get the job done. Since the enemy was more powerful than me, it was a favorable trade to remove myself from the battle in return for keeping this enemy out of it. I did this first by using my spells to delay their approach with restraining effects and difficult terrain, and then finally by standing in the entrance to physically block them from running past me. Each turn, my main decision was "How can I make it as hard as possible for this enemy to make it inside the warehouse?"
When I keep my character's motivations in mind, making decisions in each round of combat becomes much more straightforward, because while I theoretically have many options available to me at any given time, usually only a few of those options would actually advance my character's current objective. In the second example above, I didn't have to go through my entire spell list, I just had to look at spells capable of restricting movement, which narrowed down my spell selection substantially. In addition, unless my character's current motivation involves a long-term goal such as "conserve spell slots for an upcoming major battle", usually I only need to consider what my character will do this turn instead of planning ahead beyond that. Given the chaos of battle, it's rarely useful in practice to plan further ahead than 1 round anyway.
Embrace mistakes as legitimate character choices
One principle that flows from the above is that a sub-optimal choice is still a legitimate character choice. Within the fiction, characters have at most 6 seconds to decide what to do in a given round. With that constraint, it is expected that mistakes will be made, often. And that applies even to characters like mine, who aspire to be five-dimensional chess masters out-thinking their opponents at every turn. In some sense, optimal play can be considered meta-gaming, because your character doesn't have 2 minutes to decide what to do. So if you make a quick decision that turns out to be a mistake, commit to it and say "yes, my character made a poor decision in the heat of battle and has to live with the consequences". Recognize that from the perspective of telling an interesting story about your character, this "mistake" has not ruined anything at all.
I can give another example for this. In a fight with another powerful spellcaster who had a special ability to phase through solid matter, my turn came up while the enemy was underground. I made the tactical error of readying a spell to hit them when they emerged, instead of keeping my reaction open for counterspell. When they emerged, my spell hit them, but then they immediately counterattacked with a powerful spell that nearly killed my character and broke his concentration on the spell he had just landed before it was able to have any effect. Even just thinking about the character's motivations as described above, this decision was still objectively a mistake in hindsight, one that I might have realized if I had taken longer to decide what to do.
While I did beat myself up a bit for this mistake, I reserved most of the self-flagellation for my character, who was likewise disappointed in himself and had even prior to that fight been doubting his qualifications as an adventurer for other reasons. The main reason was that he never decided to become an adventurer in the first place; it just sort of happened incidentally. This mistake became the straw that broke the camel's back and brought the issue of his self-doubt to a head, leading to a genuinely interesting and heartfelt character moment in which another character (the aforementioned romance interest) reassured him and eventually led him to finally affirm the fact that yes, he is an adventurer now and not just a scholar who has gotten very badly sidetracked.
While I had this self-doubt character arc in mind for some time, I had no plans to bring the issue to a head in this way. But as I considered how my character felt about this nearly fatal mistake, I realized it was the perfect opportunity to advance this character arc. This satisfying resolution to the character arc would not have happened, or would have at least happened very differently, if I always took as much time as I needed to make the "optimal" choice on every turn of every combat.
You filled out that character description for a reason
Obviously it's possible to take this principle too far. That's how you get things like "My Guy" syndrome. However, in cases where the player doesn't know what to do, it's perfectly valid to let the character decide instead. To this point, remember that your character sheet has a list of bonds, traits, ideals, and flaws, on it. If all else fails, go down that list until you find one of these character aspects that applies to the current situation and use it to make your decision.
You can mix and match optimality-based and RP-based decisions
It should be noted that you don't have to completely abandon the goal of optimal play in favor of role playing. It's perfectly valid to combine the two. For example, suppose you are able to narrow down your options to just 2 or 3 potentially optimal choices, but you just can't make your final selection between them. As soon as you realize you are unable to decide based on optimality, use RP-based decision-making as a tiebreaker by asking yourself "Which of these 3 options is most appealing to my character?" Conversely, you might start out by coming up with 2 or 3 things your character might want to do and then make your final decision based on which one you think is most optimal.
Re-frame your decision-making using your character's "default" combat action
This is only tangentially related to the above, but it might help in combination with it. One reason that playing a fighter might feel simpler than playing a caster is that a lot of the time, the fighter just takes the attack action, so the main choice they need to make is who to attack. The attack action serves as the fighter's "default" action in combat. However, spellcasters also tend to have a default combat action as well, though it might be less obvious. Typically a caster's default action is a damage-dealing cantrip such as fire bolt. (For my wizard, it's ray of frost.) It's totally fine to fall back on this if you can't figure out what you want your character to do.
However, you can also use this default action as an anchor point in your decision making process. The completely open ended question of "what should I do this turn?" can be overwhelming, but you can replace this with a more restricted question: "Do I have anything I can do that's better than my default action?" By re-framing the question in this way, you only have to compare 2 options at a time: the option you're considering vs. the default. This can allow you to quickly narrow down your choices and make your decision more manageable as a result.
Assistance from the DM & other players
If the player agrees to adopt the above strategies, the DM and other players can help them along without meta-gaming or taking away the player's agency by simply reminding them of the relevant questions:
- What is the most important thing for your character to do right now?
- What spells/abilities does your character have that will help them accomplish that?
- What spells/abilities does your character have that are better than their standard attack cantrip in this situation?
In addition, encourage them to talk through their logic out loud, since this will give you immediate insight into when and where they get stuck in their decision making. For example: "This knight is dealing a lot of damage, so I want to charm them to stop them from attacking. Oh, but we already know they're immune to charm, what else do I have? Maybe I can trap them? Do I have any spells that restrain? ..."
(Actually, all players should talk through their turns anyway, because explaining their rationale helps the DM recognize when they're acting on a misunderstanding that needs clarifying, e.g.: "I'll move over here to get in crossbow range-" "Actually there's a wall here, so you won't have line of sight from there.")
Allow some time for adjustment
When the player first adopts this RP-driven approach to combat, they might initially need more time to decide their turns, since they will have to form the new habit of getting into their character's head during combat. However, this should generally only be an issue on the first round of combat, or when circumstances change drastically, since most of the time a character's goals don't change substantially from one round to another, so they generally shouldn't need to re-evaluate their character's goals every single round.
Summary
If I had to summarize my experience on this topic, I would boil it down to this: find techniques that help you narrow down your options to a small, manageable set of choices that are relevant to the current situation, such that you are no longer overwhelmed by the choice. You might be able to do that using the suggestions I've given above, or you might find a different technique that works better for you.
