After getting the STM board reprogrammed with a bootloader I realized that was still quite a bit of work.

In investigating boards for my 3d printer there are a plethora of 32 bit cheap boards out there now with step-stick style drivers which are designed for easy upgrades.

I've found many Step Stick format boards that pass through Step Direction and Enable to external pins. But often these 32 bit 3d printer boards are 3.3v and sometimes low current, which may not be enough for my CNC drivers. So I'm now working on a buffer board that will take Step Dir and En, level shift them to 5v at up to 16mA, and put them out on the motor pins to be connected to an off-board driver with its own on-board optoisolator, which likes 5v better than 3.3v. 

The other half would be making the end-stop inputs tolerate 12 or 24v. The easiest path to this seems to be a commercial optoisolator board, or 2, depending on number of inputs.

Then there's the 0-10v for the motor drive, but those should be easy.