Instead of hunting for the least stretchable brake cables or polishing what turn out to be shift cables, simply true the rims and adjust the brakes so the pads are very close to the rim. This will give that super firm feeling when you apply the brakes. I try to have the pads start applying braking at 1/4" of movement at the lever tip.

I used to drip oil into cable housings then grease the ends, on the theory that this stopped water from getting into the housing. Cables used to actually rust, back in the day. Don't think I've done that for a couple decades.