I don't do any type racing but am curious as to the need for a frame bag in a gravel race?
I ride my tourer, and sometimes my road bike with a smallish Banjo Brothers bag. I picked a (cheap) pair of running pants when my knee kept ticking against a small overlap of velcro. I trimmed the velcro and problem solved. My knees don't rub my bag. I think that would bug me. I also don't overstuff my bag so that could possibly be your problem. Or it simply could be your position on pedals or how we're built. I don't think a frame bag should rub your knees. You said it rubs "just a bit" so it could possibly be corrected with a little tweaking of the position or something. Dunno, but like I said, I think it would bug me if it were a continual thing.