Not fixable, easily. Just has crap PWM control, that is so bad.
No tool changer?
I just run the spindle manually at the speed I want.
As a comment with a tool change, saying tool size, I make a comment as to the RPM.
I gave up trying to fix bad.
There are quite a few other threads about this problem, giving plenty of insight.
Search SX3 and SX4 spindle.