You got most of them, but you failed to consider magic items, you missed a couple of spells, and you were only thinking of creatures
There are two classes of targets that we want to consider here:
For objects, we obviously can't impose a condition, so we need to look at magic to help us here.
Fortunately there is a Magic Item called
Shatterspike (from Out of the Abyss)
[...]. If it hits an object, the hit is automatically a critical hit, [...]
Alternatively, from XGtE, if you are in possession of an Adamantine Weapon it will auto-crit objects when it hits.1
If a creature is wearing Adamantine Armor you can't crit it, as the armor turns critical hits into normal hits.1
As you've noted the first port of call is to look at imposing a condition on a creature. The conditions which enable an automatic critical hit are:
- Paralyzed (auto-crit if you hit)
- Unconscious (auto-crit if you hit)
How can we impose these conditions?
Lets address Unconscious first:
- Reduce them to 0 hit points, they are then unconscious and you can start wailing on their body
- Cast one of the following spells:
- Charm the single enemy, and then cast Astral Projection to bring their bodies on a journey with you. Their bodies are now unconscious, and your party can score critical hits against them.
- Have/summon a pet monster that can impose the Unconscious condition
Now lets look at imposing Paralyzed:
- The following Magic Items can impose the Paralyzed condition:
- The following spells can impose the paralyzed condition:
- Have/summon a pet monster that can impose the Paralyzed condition
Additionally, if you are an Assassin Rogue and can get surprise on a creature then you can auto-crit them when you hit. This is a one turn per combat feature, but it is very strong.
Finally, if we can turn a Creature into an Object, then we can score critical hits against it's object form with the aforementioned methods. Whether or not scoring those criticals is useful is beyond the scope of this answer.
How do we turn a creature into an object?
You cast True Polymorph, or kill them.
If you rule that a petrified creature is an object (i.e. they are both an object and a creature simultaneously) then casting Flesh to Stone or having a pet Medusa/Basilisk petrify them would also suffice. See this Q&A for more on that debate.
1: Thanks to @Pilchard123 for these two references