These are some good basic points to have so far! I can't offer too much in the way of CR since Luke dropped out of college and he's about 6-7 years younger than Augustine, but I can offer some help with fleshing out his AU if you'd like?

One of the things that might be good to consider is Augustine's motivations to explain how he got from point a to point b. For instance, why did he decide to move to Paris to study? What lead him to studying animal biology?

Other ways to shape him would be thinking about influences around him. Is he from a lower class, middle class, upper class? Does he have any siblings? How close is he with his family? He's a bachelor who flirts sometimes, but does he ever think about settling down someday?

As for his personality, what's he like in canon? A great way to flesh out his AU personality is to look how he's treated canonically in the anime and what traits he displays and then incorporate them into your AU. Is he very friendly with people or generous? Part of your AU could be that he has made a lot of contacts while he was studying in France and in his spare time in Recollé he volunteers at the animal shelter. Little things like that can help create a fully fleshed-out AU that allows you to grow your character.

I hope this kind of helps a little!