Ideas:

1. Mach3/4 USB breakout boards often run an STM microcontroller, reprogram it and make it run GBBL locally instead and then use the USB input for GCODE transfer from another device, or hook to something like an ESP to feed wirelessly.

2. Cypress PSOC5lp has a nice, cheap carrier board and runs at 5v. Make the worlds simplest interface board and break out pins to 26 pin IDC for connection to 25 pin Dsub. Hook up to easily available CNC parallel breakout board and run maybe GRBL-HAL. It looks like there are enough pins to do 2 breakouts, maybe use the second one for more inputs with a cheap commercial opto board instead of a parallel board.

3. Same as 2 but with other simple carrier boards(STM nucleo/bluepill, ESP32, TI, ?), maybe more complex based on 3.3v interfacing.