Part cooling fan speed
You do not need to replace the stock part cooling fan to have variable speeds. The solution to this is to edit the MKS_THR.cfg file and find the [fan] section and edit the stock values to match the below.
[fan]
pin:MKS_THR:gpio1
max_power: 1.0
kick_start_time: 0.5
off_below: 0.01
cycle_time: 0.05
This will mean the fan will operate properly and you should be able to change the speeds. I hope this helps!