I really like your regulus take and was wondering if you had any other ideas?
I have a few different iterations on Regulus depending on the story I'm writing.
The whole 'Regulus is his father's favourite son' is extremely vindictive. He sees Sirius as a coward who left him behind without a thought. So he's angry and wants revenge on his brother. This Regulus is definitely a chess player. You think you've got him but thats what he wants you to think. He is ready to face everything if it means hurting Sirius and inadvertently hurting himself and the people who care about him. Hes almost Machiavellian but he can turn back before it's too late. Its almost the question of does he want to?
However, the Regulus that runs away and sprouts Shakespeare to Barty is someone who doesn't want to think about his family. He is not just physically running away but mentally. He will happily play pretend with Barty as long as Barty is with him. This Regulus is overwhelmingly relying on Barty emotionally which isn't great when both of them are trying to work out who they are individually and romantically.
If the first story is about revenge, the second is a coming of age story.
(The Prince story was written because I was bored and I had just watched House of the Dragon so no thought went into that.)