I don't think it's true that a mid-frame hinge causes flex. There's flex in Brompton's hinges because of the particulars of the design but in other bikes I doubt there's any significant flex originating from the hinge. Furthermore I think on small-wheeled bikes frame flexibility is desirable as it absorbs shock/vibration. It's the stem/handlepost flex that's the problem and it looks as though this Big Fish would have plenty of that.