I'm not 100% sure what fixed it, but I deliberately tried to press the power button in off angles (i.e. on the side of the button instead of head on) and with different frequency intervals to try to make it unstuck and after a few minutes of doing that it suddenly stopped restarting. I think it was the power button that was stuck, even though it wasn't physically noticeable, so probably something is disturbing the power button further in than the actual button - but since the phone is both hard and expensive to disassemble for me, I won't know for now.