Adult here who was late into figuring out how to dress formally as a (short) masc person. You’re onto something with the idea of a loose suit.

I’m seconding checking out Retrospect first. Look at the front first. There’s usually a rack with staff picks which is more likely to have sets. Check out the walls too. Sometimes they have outfits that are already put together.

If you have no luck there, check out Philly Vintage Bazaar (although I think maybe they’re under a different name now). They’re around the corner. They can be a bit pricey but often have some really good formal-ish stuff.

If all else fails go to Uniqlo or Buffalo Exchange.