I found the braking improved markedly after scouring through the CD anodizing, on my Mavic EX 721 rims.

Having Magura HS 33 Hydraulic Rim Brakes , a couple winter wet down hills with their relatively Abrasive
Green pads was enough..

then I went back to using the pads they got Kool Stop to make for them ..

HS pad holders are a literal Snap to swap out, from the end if the slave cylinders.


Exercise bikes use Felt pads to increase the drag, those would win the softest criteria.