This is what you want. Same concept, open firmware, but split.

https://www.gboards.ca/products

https://geekhack.org/ "Interest Checks" and "Group Buys and Preorders" typically have a few running\comming up splits.

Check out UltimateHackingKeyboard [0] for a developed, split, open-source keyboard. Can definitely recommend.

[0] https://uhk.io/