The risky part where the speed is important will last for about 1 minute each time so it will be importand to know if they are running every... Say 3-5seconds or so, I don't need to measure the rpm from all motors.
I can do like a hall input change on 95% and rpm calc on 5%.
The cheapest in my opinion will be muxing then, can you all recommend this?
The good thing is that I don't need any communication.