so spoilers(?) for Clouds of the Soul because I was inspired and also impatient. Thomas will get to live his agender he/they truth in the (distant) future of that fic. Initially, Vincent buys some clothes for him to experiment with. All of these are based on actual clothing references I've found for them.
Trousers with back/side panels so it's a trouser/skirt combo (there are a few different designs of these and they are very sleek and smart. I feel like they'd suit him. Thomas should be able to experience swishing without a cassock).
Short, front fastening dresses worn as open overshirts.
Generally they should have lace in their life too if they've never been a priest to wear such things. So lace trim shirts etc
(There's a tiny Gender Euphoria Thomas from Collar Me, and the collar, because I had space to fill. Ty to everyone who loved and supported an agender thomas in my pride picture. If you’ve not read this fic and just want to read his agender euphoria moment without 170k of character development to get there, you can just read chapter 14)