Hi mate, grab a file and file/sand off the paint off the rear rim. If you’re struggling for decent pads a bit of tar will definitely help. Next step would be to get yourself some decent pads (I’ve used yellow and blue heatsinks and black adm pads and I’m happy with any of those). As Ross mentioned putting a brake booster on is another thing to consider. Stops the bike from bending and protects the hoses cos they snap quite easily if you were to accidentally step on them. Finally to make the brake feel stiff try to adjust the callipers/pads to be perfectly parallel to the rim. You’ll also notice there is a wee hidden screw towards the end of the lever blade. Screw that in to bring the blade closer to the bars if you have short fingers like me. Would definitely recommend upgrading to metal tpa and a metal piston. I’ve always water bled my brakes and they seem to work ideal.
Have a look at Ali Clarksons latest vlog on YouTube for other tips and brake adjustments.
I don’t really know much about disc brakes but cleaning the rotor with spirit should help.
I can only assume you are happy with the angle the levers are set at but might be worth trying out different angles of the levers and bars. Reaching that far down for the lever might be placing pressure on your palms which is making them hurt? Just a thought.
Could grind your rear rim, and try metal backing pads too. The stiffness may help or hinder, so worth a try. If it helps, try running a 2 or 4 bolt booster for added stiffness.