Sometimes smaller is better. Thanks @Thomas for eForth on STM8. I've added the files to this project where I have tested the concept of using STM8 running eForth to be the controller for the RB3DP. The forward and reverse kinematics routines are tested, and a few GCODE commands are implemented. Stubs are used for actual actions, with text echo of internal commands.
Based on this test, RB3DP will accept GCODE in millimeters and have a maximum size of about 60 meters (the 16-bit integer maths maxes out at 65.535 meters).
Now to put it all together on ESP14 (which is STM8 with ESP8266).