Rear Brakes Wearing Poorly, Noise


My 2018 Mazda 3 5 door is having some rear brake problems and I was wondering if anyone else here had a similar issue.

Since almost new (about 11 months ago), the rear brakes did not show even wear on the rotors. Not even a smaller stripe of worn rotor in the middle, just a bunch of lines - nothing even. Starting about 6 months ago, they also make a rotational scratching sound that is audible at lower speeds.

My previous 3 - a 2005 - also wore the rear rotors unevenly. Is this a known problem?

Getting to the dealer is a hassle, so if it's a basic re-greasing of the caliper slides, I'm inclined to just deal with it myself.

Any other ideas?
2014 Mazda3 S GT auto, 2008 MX5 6-speed
My coworker's 2018 Audi had circular grooves in its rotors. It seems to be more likely if the vehicle traverses dry, sandy roads, as grains of sand can easily score brake rotors. Can you post a photo?