I have a vintage peugeot mixte that would have room fenders and wider tires, 38mm maybe. It's heavy but since it was a Canadian made peugeot I think it has standard threading, so you could likely update one of those easily, but they are heavy and have really relaxed geometry; I have no clue how it'd handle as a touring rig but it served me well for a few weeks of winter commuting last year when I was in a pinch waiting for parts.

I don't think I've ever seen an aluminum mixte but a quick google led me to the specialized live 2, a mixte frame that has bent middle stays, but I guess it's still a mixte. Linus also offers a mixte bicycle but I dunno if they sell framesets or anything like that. Might be better off going custom or hunting down a high quality mixte from the 80s.