Um...
I have to really take a deep breath and be thick skinned here, but seeing how much you love PCB design, would it be possible to ask you to design a 42 key PCB that basically puts together your Six Shooter and samwisekoi's GH36?
I have been wanting to get a 6x6 matrix with 2x3=6 thumb keys for a long time. Essentially an ergodox with matrix layout and thumb keys located nearby.I really can't do this design myself, the GH36 doesn't completely meet my need so I would have to hand wire additional keys, etc.
An thinking that one way to make it easier than the Gh36 would be to forego LEDs. Another would be to leave out support for additional stabilizers or Alps switches, making it a pure 42 key matrix. A third could be to have 1 Atmega per hand without having to run a wire between the two sides. That way each keyboard functions independently and can work as a one handed board if the thumb triggered a function layer for one handed operation.