Many mid-90's or so mountain bikes have 68 mm BB and steel frames with a 1 1/8" head tube for a recent, straight-steerer suspension fork. I have at least two in my "stable". Look at CFG (on Amazon); cheap steel frames 26" & 29" (when they're available) with 68 mm BB.
Not sure it's what you're looking for, but I know the entry level aluminum treks from the late 2000s were overbuilt, strong frames with 68mm bottom brackets. Models like 4300, 4500... Later called "series 4" . Could probably get a frame, or whole bike dirt cheap now.
this would be pefrect for mid-drive (it's a modified gng 2015, up to 1500watts, however considering it's rated 75% efficiency, and we have a jackshaft and primary and secondery reduction, i'm not sure if the wheel gets 700 watts)