I think percentages to hit are fine. As already said, players figure out AC's of creatures quite quickly anyway.
It also has the added benefit of letting people unfamiliar with DnD know how likely they are to hit.
Having to look at enemy animations to figure out what AC they have sounds cool in theory but will become tedious. At least for me.
Not to mention having to know how each animation looks like on different creature and the need to create those animations in the first place.
Plus, whenever I run DnD I tend to "highlight" AC by describing the enemy a bit more, particularly if it is not humanoid. That is absent from the video game, since all you are relying on is the player looking at a model and making the conclusion themselves.
The goal is that, after you've faced an enemy once or twice, you pass your nature/arcana checks and the enemy's AC is added to your bestiary. At this point, the percentages to hit are shown. It's only for your first fight against a type of enemy that you don't know the AC (for these purposes, all goblins could count as the same creature, even the goblins with differing ACs).
So you won't have to look at enemy animations for most fights.
I don't know how much work creating animations is or how easily one creature's animation can be copied over other creatures, so can't really comment on that. But that's a cost-benefit analysis that should be done by Larian, not us.
This is a very complex solution. How would you animate this so accurately that the player can see it clearly. This would also require more animating every single unit which seems set in stone already. Devs could do what you said by writing everything in the combat log but then you would be checking combat log the whole game and not pay attention to animations. -snip-
My idea is that a "miss" or "blocked" or "dodged" text would pop up over the enemy along with the animation. It also doesn't require animating every single unit. At the very least, all humans(+elves/tieflings/etc) can use the same set of animations.