Could be the combination of stem length/type, riser bars and headset all contributing. If the handling is spongy with that combo, discs aren't going to help, they're going to make it seem worse. Sounds like the problem lies outside the discs but the discs don't help matters.

Personal bias: I'm not a fan of discs except in one circumstance. If you live or travel in really steep hilly or mountainous country, discs prevent overheating the rim during long downhill braking, preventing blowouts from overheated rims, which can be quite dangerous. While discs have superior modulation compared to rim brakes, one has to keep the pads & rotors meticulously clean to avoid excess wear, and most folks don't do that. I'm perfectly fine with rim brakes on my rides, folders and non-folders alike.

I'm thinking that NWT is not the bike for you. A test ride on a different BF is recommended, so you can get a baseline feel for what a properly set up BF rides like. It's not always 100% like your typical road bike - despite what BF says - but it gets a lot closer than the one you rode today.