Keyboard only ...well almost

Ratpoison
https://www.nongnu.org/ratpoison/
Ratpoison is a simple Window Manager with no fat library dependencies, no fancy graphics, no window decorations, and no rodent dependence

Carpalx
http://mkweb.bcgsc.ca/carpalx/
Save your carpals and reach typing guru status with your own keyboard layout, such as the fully optimized QGMLWB layout. Alternatively, use your power for evil and create a carpal-breaking TNWMLC layout to embarass office prima donnas and punish sluggish workers. And it's the only layout with its own line of Brazilian clothing.

Next
https://github.com/atlas-engineer/next
Next is a keyboard-oriented, extensible web-browser designed for power users. The application has familiar key-bindings (Emacs, VI), is fully configurable and extensible in Lisp, and has powerful features for productive professionals.

DAS-UI
https://das-ui.firebaseapp.com/
DAS-UI is a domain-agnostic keyboard-based visual programming language with JavaScript backend.

Multiple Keyboards stuff
https://superuser.com/questions/390458/multiple-keyboard-and-mouse-inputs
https://forum.unity.com/threads/multi-input-handle-many-keyboards-mice-and-pads-all-at-once.396770/
https://wiki.archlinux.org/index.php/Multi-pointer_X
https://ice-wm.org/ (...with xinput library (maybe with Mate Desktop ;) )