Thank you very much for your detailed explanation! Much appreciated!
I actually really like your first option!
The second I agree with has this built in problem that automatic numbers would change, as you say.
I like that if you are not using this feature (your first suggestion), everything is just looking fine and simple. Then if you are advanced enough to need it, then you can just take it into use. And because the drawings are often so similar anyway, having a symbol (letter) occluding part of the x-sheet thumbnail doesn’t change anything – it will only help.
This is going onto our todo! Thank you VERY much Pharan!