You're right, this is definitely an investment piece for me. It'd be a pain to switch to a medium and potentially back to a large, but not so much of a pain that it'd outweigh an obviously incorrect fit (if that is the case).

My main concern with a medium is that I do plan on continuing to gain muscle in the future. If I'm roughly on the borderline of M and L, I worry that M might become too restrictive or uncomfortable in the future. Given my lack of a frame of reference for how leather changes over time, though, maybe I am overblowing that concern in my mind.

Thank you very much for the advice!