You may want to try a different approach to how you manage role playing and alignment.
Caveat: Whatever you take away from these answers, discuss it with your DM. Let them know how you're thinking of playing your character and make sure you understand their policies on paladins and alignment. (and it probably wouldn't hurt to know their policy on bringing on a new character)
First off, your question mentions being hamstrung by the DM in your actions. Ideally, the DM has no control over your characters actions (assuming they are not simply physically impossible). All they should control is how the world reacts to your actions. Your character is a human; flawed and limited. You are absolutely allowed, even expected, to make mistakes and occasionally commit acts unbecoming of a paladin. The conflicts and consequences of such actions are the stuff of great drama and role-playing opportunity. Paladins occasionally fall; it's why we make their pedestals so high. Don't be afraid to act for fear of contradicting your alignment.
You mentioned you thought of your character as a Robocop style knight. Let that template be your guide, and focus on what that character believes is right and necessary. Don't worry about your alignment per se. Though you are a paladin with a rigid alignment restriction, alignment is not a fixed quantity, and it does not define your actions. In fact, the PHB tells us it's the other way around:
It is probable that your campaign referee will keep a graph of the drift of your character on the alignment chart. This is affected by the actions (and desires) of your character during the course of each adventure, and will be reflected on the graph. You may find that these actions are such as to cause the declared alignment to be shifted towards, or actually to, some other.
Alignment is a function of what your character does. Rather than thinking of your character as a Paladin, it may help to think of them as a person who is trying to be a paladin. Hopefully this will help a bit with your role playing.
As for dealing with the troublesome character, go back to your framework, your idea of who your character is. In this case, that's Robocop. The nice thing about Robocop is he actually came with a short, specific set of rules!
- "Serve the public trust"
- "Protect the innocent"
- "Uphold the law"
(We'll ignore the spoiler 4th law for this discussion)
So, when you need to react, try to frame your response from these. For instance, let's take the case where you got fireballed by your reckless teammate.
Serve the public trust: Your character would likely not hesitate to sacrifice themselves if it was necessary to defeat evil, so maybe you wouldn't take the hit personally. However, you are a force for good, and your power should not be removed forever from Tyr's service except under exceptional circumstances.
Protect the innocent: If your wizard had done the same to a less beefy character, they would likely have killed them. You have a duty to ensure that your comrades remain safe, and if the wizard's carelessness endangers them, you need to react.
Uphold the law: There are probably few written laws in your setting, so you probably can't arrest them for assault. However, there are usually codes of honor among companions, especially warriors and military types, that involve safeguarding your comrades, as I mentioned above.
To actually frame a response, you need to do 2 things:
- Tell them what they did wrong, and
- Administer either punishment or a warning of impending punishment for continued infractions.
As you noted, punishment is tricky, since typically no one in an adventuring party has the authority to do so. A few thoughts:
Issue the warning, "If you recklessly endanger another party member again," followed by the consequence:
- then (some portion) of their next share of loot will be split amongst the other party members. (This will provide monetary incentive to be careful, and splitting the fine up with the rest of the party will get them to back you up in enforcing it.)
- the wizard will be asked to leave the group. (This is a tough ultimatum, and not every group will take this well, so be careful with this one. If the wizard doesn't step in line, and the rest of the party doesn't back you up, then your character may feel like they have no choice but to move on. Or they may surprise you and back your play, in which case the wizard may get the boot. That can be perfectly fine, by the way. Especially if the players are ok with the story evolving that way. As you said in your question, changing alignments and classes is one of your options. Allowing a character to ride off into the sunset and letting the player re-joining the game as a new character should also be a valid option. And it removes any in-game animosity between the characters if one leaves. Not to mention the fact that disgruntled former comrades are a DM's goldmine for future recurring villains!)
- the paladin cannot sanction the casual disregard for innocent lives, and decides that this behavior places the wizard outside of Tyr's protection. Therefore no Lay-on-Hands healing, no tanking to block foes from engaging the wizard, and cessation of all other services, clerical and martial, until the wizard demonstrates that they can act as a civil companion and can be trusted by the Fellowship of Tyr.
or, there's one other response that comes to mind, albeit from an arguably non-LG character, and that's Simon Tam of Firefly. That might go something like
- "You have been reckless and careless, endangered my life and will likely endanger others in the future. And if it continues, know this: I will never harm you. I am this party's defender and protector, and by Tyr's might I will do everything in my power to save my companions. And I expect they will do the same for me. For me to do anything else, to live in fear or constantly have to watch my back, that would weaken me and distract me from my duties to my god. So I've decided not to go there, and instead put faith in your understanding that we are better served fighting side-by-side than against each other. But, of course, the choice is yours..."
Now, I've had unlimited time to think this stuff up with no pressure, but hopefully these kinds of examples will give you some seeds for thought, or at least get you thinking about what leverage you character might have and how they might apply it to uphold "decent" behavior.
One last note: If at all possible let the other players know your character's dilemma. You want to make these kinds of potentially explosive social interactions part of the game, not a personal player argument to blow up your gaming group.