Here is the best setup (I think so :D) for K-series Keychron keyboards on Linux.
Most of these commands have been tested on Ubuntu 20.04 and should also work on most Debian-based distributions. If a command happens not to work for you, take a look in the comment section.
Make Fn + F-keys work
Keychron Keyboards on Linux use the
hid_apple driver (even in Windows/Android mode), both in Bluetooth and Wired modes.
By default, this driver uses the F-keys as multimedia shortcuts and you have to press
Fn + the key to get the usual F1 through F12 keys.