but i did something similar last week and so started reading the thread. Interesting discussions.
Since you are using raspberry pi4, have you though about using astroberry-diy focuser? I have not used this module, but from what i can gather going over the code, if requires one to hook up a motor driver to the pi, and use the indi-astroberry-focuser to control it. This driver allows you to select which GPIOs the motor driver is connected to. so i imagine it is a matter of getting a DRV8834/DRV8834/A4988/A4988 and hocking up the cables properly.
I did it somewhat differently, in that I had the motor driver, drv8825 in my case, connected and fpga and connected that to the indi by using a indi_fpga_focuser derived from the above-mentioned indi module. Please see the screenshot.