Since most of your braking comes from the front brake, you could opt for a disc brake up front and a rim brake in back, or get an IGH with a drum brake. That way, you don't have to modify a frame, just find an appropriate disc compatible fork. I don't have any experience with belt drives, so I can't comment on them.

As it happens, I just completed a build for my S.O. by modernizing a vintage mixte. I did a write-up here (it's the second bike in the post):

ROAM: Winter Projects