Remora - Rpi Software Stepping Using External Microcontroller via SPI

its cheap but sure looks very limiting.
When there was not a shortage i only paid a bit more for the skr1.4 with the lpc chip.

The thing with the STM32F401 boards are that they are cheap and a simple form factor that would make it easy to make a break-out board for driving  seperates stepper drives and have encoders and inductive limit switches etc. the form factor of the bigtreetec boards are great for printers but not so great when you want to control milling machines and lathes etc.

Same with the ESP32, although I am not sure if it has other limitations that make it unsuitable

Great project by the way though. 

You didn't post a link or a part number for the dev boards. Do they bring all the chip IO out to pads on the board? Beware of clones of knock-offs as they skip all sorts of things that they might obfuscate in the specs or even pics.

Here are the boards I had in mind...
www.aliexpress.com/item/1005001798664558.html
which should be laid out according to the image attached here... I see there are 2 versions F401 and F411, the F411 probably the better option as it runs at 100MHz
image missing https://miro.medium.com/max/3122/1*ixFS8iYk_uO_nWkH0w6jFA.png

Just limited IO. Make count of what you need. It has more IO than a typical LPT port.

Certainly enough to run a 3 axis mill, spindle speed and endstops. I have a small desktop lathe I want to CNC. Mechanical bits are done, ballscrews installed just need the control portion.