He looks pretty good for his age, you've taken great care of him.
You started the thread saying you've made mistakes that you regret but to have a 15 year old dragon it shows that you've actually been a good owner and made mostly good decisions for sure.
You're getting good advice so far, my suggestion about not being aggressive with his treatment was just to be ware that vets aren't using methods that hurt him in his fragile condition.
Hopefully he will live to 16 or ?? years old. So far so good . I think your enclosure is fine and you don't need a larger one, especially since he's old.