Sources:
- https://raspberrypi.stackexchange.com/a/66324
- https://raspberrypi.stackexchange.com/a/72518
- https://github.com/avanc/mopidy-vintage/wiki/Automount-USB-sticks
So, I found a solution that works quite well. Big thanks to avanc and his udev rule that makes this possible. I also modified it so that it could mount up to 4 flash drives at the same time (it can be increased if needed).