Then I designed PCB for one switch. It is with breadboard friendly layout and uses through-hole components. And made a plywood mounting plate, stained it black and lacquered. I connected the buttons through the mounting plate to the circuit boards.
Then I figured out which buttons I wanted and soldered the right C and R wires in the right places.
I also made a plywood case.
The next part was keycaps. I could use plastic ones. But I wanted them to be symmetrical and with symbols on them. So again I made them from plywood. The top layer is solid oak. Other is birch tree plywood. Laser cutting them and glued together. One part is 3D printed. Cross shape part that connects keycaps to switches. The hardest part was sanding the keycaps to the right shape. I did it by hand, but it should be a mechanized process. And also my keycaps are larger than normal keycaps on the keyboard.
It's beautiful and I'm especially loving the wooden parts (I'd like to find a less labor-intensive way to make my own).
Regarding the electronics, one could also use an Adafruit Feather nRF52840 Express + battery to make this a wireless Bluetooth solution with an easy bit of Circuit Python.