Yes, nayax monthly fee is per device.
I've had 12x nayax in bays uncovered for 3+ years and another 8x for 1.5+ years without issue besides customer abuse. They can handle the car wash environment.
I'm not promoting nayax over the others, but my personal experience with having 30+ installed in self serve car washes has been good. Everyone is going to have different opinions on what is most important to them in a
credit card solution, and should choose accordingly. Nayax is a global fintech company which brings its own set of pros and cons beyond just their devices