Interesting that you bought, and waited so long to correct, what seems to be an ill fitting bike. I can understand that it takes time to realize that a particular handlebar config is comfortable/uncomfortable, but proper seat position/leg extension should be known day one.

FWIW, I do some touring on the Brompton, and other bikes, and personally don't find one handlebar position - whether S-bar low, or M-bar upright - to be particularly comfortable, but rather simply the ability to vary between the two, back and forth while riding to be more comfortable (with multi-position drop bars most comfortable). My Dahon has an adjustable height handlebar, but it just doesn't do the trick since you cannot vary position while riding, and frankly I find it an annoyance as another point of looseness/slop and another step in the folding process.

I personally got tired of my M-bars upright position and aerodynamic drag and so rigged a drop bar grip position in the bottom my M-bar's 'U' that has resolved both issues for me. This upgrade has made me virtually indifferent to riding my B, or 700c gravel bike, on regular rides (no folding involved).

As much as I love my M-bars now with 2 riding positions, and 3 grip positions (GP2 bar ends), if I where do it again, I would seriously consider the P-bars. Don't care how ugly they look, we ride 'circus' bikes after all.

Good luck on your upgrades!