Generally speaking the best speed for aluminum is 8000 rpm. Thats what my tests show. 25 ipm feedrate is good feedrate to start with. No coolant or soap water or sprayed alcohol via airbrush.

I believe the problem is in the VFD, as with 0.8kw spindle i have no problem at that depth and feedrate / my machine is not strong/.

Torque boost is for very low speeds. The VFD should be set to Constant Torque.

Single flute bits work best if speeds are problem.