There is Edgar Matias's "Half Keyboard" layout, where the right hand's keys of a regular keyboard are mirrored on the left hand when holding a modifier key with your thumb. The idea is that if you have learned touch-typing then muscle memory for the right hand should be available also on the left.

Matias wrote an article [1] about it and then made it into a commercial product [2], but the concept should be possible on any programmable keyboard. Perhaps it would be possible with a AutoHotkey (MS-Windows) or Karabiner (MacOS) script otherwise.

There is a large scene for more-or-less DIY "ergonomic" mechanical programmable keyboards with various different physical layouts, but common themes are 1) that they are split in a pair of two physical keyboards and 2) that they have multiple thumb-keys for modifiers / Return / Space. You could build and program just one half of such a pair. Many years ago I programmed an ErgoDox with the HalfKeyboard layout, just to try it out, and that ErgoDox I had built on a budget from mostly salvaged vintage components.

[1]: https://www.semanticscholar.org/paper/One-Handed-Touch-Typin...

[2]:https://matias.ca/halfkeyboard/