Are you sure that you don't have something like while(!Serial) in your code? This would wait for an usb connection

@Foxek Do you mind telling us which custom driver you used?

Since "the motor also stalls at high microstepping but same steps/sec. That is even at low RPM it can’t take more than 65000 steps/sec" I suspect something is wrong with your sketch (TeensyStep should be able to do up to 300'000 steps/sec on a T3.6). Can you add a minimal example showing the effect to your question?

BTW: you can also PWM the pin on the Teensy (analogWrite()) to avoid the losses in R2

re MOSFET: I often use a irlm6344 to directly connect to Teensy 3V3 pins. octopart.com/irlml6344trpbf-infineon-65874984?r=sp . Works without issue so far. Only external part I'm using is a 100k resistor from gate to gnd. It is good for 30V 5A and comes in a tiny SOT23 package.

Sorry, my comment was unclear indeed. I might be wrong, but Ib needs to be supplied by the Teensy pin. The IMXRT datasheet is very vague, but sourcing/sinking 25mA seems to be on the high side.

Ib 25mA might be borderline for a T4. Maybe a darlington configuration might be better?