The belt for driving the spindle shouldn’t be loose, it should be able to be depressed ~2–4mm so should be tightened up.
The 4 bolts holding the brushless motor allow the belt to be tensioned.
The belt does not have to be tight.
When pushing in the center of the pillows, about 2-4mm deflection is normal for the belt.
further:
as long as you’re not seeing the belt slip you should be good to go. If the belt is too tight you lose a lot of torque to friction.
To test the motor, connect to the machine, bring up the MDI and:
…type M3S1000 and hit send. This will turn the spindle on at 1000RPM. Let it run for a bit and look for odd behavior (like stalling). Now type M3S10000 and hit send. This is max RPM. If the motor is at fault, the spindle will stall within a few seconds/minutes.