Thanks for the reply. The device in question hasn't been designed for back massage specifically, I've only repurposed it, because it's the only type of device on the market that really works for me. It's small and can be operated by one person, as opposed to those big sticks that you need a second person to operate.
With the motor overheating, your idea is probably correct, as I lay on the device with my back, so its squashed between my back and the mattress. My first device kept the motor running at the same speed and the batteries were overheating. The first motor probably had more torque.
As for the hardware itself, the motor is really cheap and no writings on it are present. I assumed if I used better quality components my problem could be at least mitigated so the motor doesnt burn out after 2-3 months.
Any suggestions as to what type of controller I could use that supports up to 4 motors and is easy to program for someone who can't really write any code? Like with lots of tutorials and support?