I remember several years ago listening to Holly van Gulden speak and she talked a lot about constancy and permenance and of a child doesn’t have this (which a lot of children who have been adopted have missed out in early childhood experiences and so may have missed out on key areas of development) can really struggle at bedtime. The thought being that if they close their eyes will things still exist? If they don’t have permenance, that then becomes questionable so do what they can to avoid sleeping for the fear that things might disappear while they sleep. It’s outlined in a book called “Dance of attachment” by Holly van Gulden. I never really thought of it at all until I read your post. I wonder if that has something to play? Might be worth reading about.
My son (adopted) is a really good sleeper but if he has had a happy day, he might delay bedtime. We stick to a very concise routine because dragging it out drags out the separation and abandonment he feels at bedtime. We go through spells of sitting outside his bedroom door, legs extended and door open, so that he can’t make eye contact but see our feet, and read or something so he knows we are there. He feels much more secure knowing we are nearby.
Also, does he have a transitional object that smells of you? A scarf or your pillow? That might make him feel closer to you.
Terms of wetting the bed, I’d just start putting him in a pull up too. I’d also have a spare duvet, made up so it’s a quick change and don’t say too much.
Have you tried audio books? You could even record yourself and play that back when your not in the room? It might be worth a try. When my son is dyaregulated, he sings and dances (which also consists of throwing himself on the floor and stamping. I think it’s sensory with him).
I don’t know if this helps but might be something you’ve not tried.