My Opel used to have two options for pads, abrasive and non-. The idea seemed to be to use the abrasive only for truing a warped rotor.

If you go buy a cheap set of new pads, then apply some self adhesive sandpaper to the elcheapos, then jack up the rear and lightly apply the brakes while driven by the engine. Quit when the chuff-chuff-chuff sound goes away.

I would anneal the disks first, perhaps relaxing some of the warp. This could be done in the home oven, given an absent or VERY understanding life partner...

Not responsible for advice not taken...