Everything about it is open source, the only limitation is microcontrollers that support circuitpython. Most of the controllers that work with QMK also work with KMK. The pi pico for example that’s used is much cheaper than the typical arduino pro mini, and comes with more storage and i/o pins.

3D print it or have it printed for you, https://www.printables.com/model/235677-logitech-mx-mechanical-key-caps

KMK is similar to QMK but it runs on python, and it being python means that nothing needs to be recompiled for it to apply your changes. The pico has circuit python running on it which acts as a removable usb drive that contain your config files. With any device that you can use to read a flash drive you can reconfigure your layers/keys with. It’s quite flexible to use. Open up notepad/vim and click save, the board reboots with your changes.