Setting the double work of the rotary axis

Good afternoon dear friends.
I have been using LinuxCNC for a long time, but I am not so familiar with it.

I want to change the logic of the rotary axis. It's no secret that there are problems when working with a rotary axis. Slow speed and so on. My idea is to make the rotary axis rotate at a constant speed without using the A axis. Yes, I understand that with a stepper motor I cannot achieve high speed and torque, but it will still be better. I just set the rotary axis to constant speed and make the machine move along the vector in the YZ plane (the rotary axis is set along the Y axis).

I am using controller 7. And two optocoupler boards. I don't think this information is useful, but still.

I managed to set up the rotary axis as a spindle for PULL DIR control. But I would like to go further and make this function independent of the spindle.

What I would like to get in a perfect way. Using some M - code, I start the rotation of the rotary axis (for example, let it be M 998) and turn off the rotation of the rotary axis (for example, let it be M 999).

Thus, in the process of executing the program, I can start both turning (not quite turning, but let's assume in this case that it is really turning) and full 4-axis machining.

I tried to cheat and through the mux function just set the rotation speed of the rotary axis net a-output => mux2.0.in0
net rout mux2.0.out => hm2_7i92.0.stepgen.03.velocity-cmd but that gave no results. Apparently the problem is that the pid does not receive position feedback and I get a tracking error.

Dear friends, tell me which way to look in order to solve my problem. Maybe I'm going down the wrong path and can I solve it differently?

If you want to use your stepper motor as spindle, you need to change mode for this axis at first, from position to velocity.
Look manual for stepgen for more details.