With most mechanical keyboards I’ve encountered the FN key is fully programmable, along with every other key on the keyboard. Typically rather than having the functionality hard coded the FN key will activate a second layer, it’s similar to how shift activated a layer of uppercase characters, but with different functionality.

On mine I have a key that flips into a symbols layer, placing characters commonly used in programming either on or close to the home row, and flips hjkl into vim style cursor keys. I’ve also got another layer on a different key which disables almost all my customisations and turns it into a standard qwerty keyboard for gaming.

If you’re deep into layout customisations via X I suspect you’d really like a decent mechanical with customisable firmware, and probably a few spare keys for mapping to layer shifts.