If you want to build up crafting skills in general, the fastest way is to do repair on MS as this gives engineering skills. If you also craft your own RK-0 tools and welding wire, you get additional skills from that and it costs less than buying supplies and the welding wire BP uses a lot of VS.
I firmly believe that while sweat cost is only included at TT and cause bigger TT loss, it still get partially adjusted by the MU and the perception that I unlock more bps crafting with sweat. The bp part may not be right but many seem to notice that. Still, I find players + few years of experience not enough to be 100% sure as most ppl I know dont craft only sweat bps making it small sample imo even combined.