Here is the best setup (I think so :D) for Keychron + Linux
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.
In order to change this, you need to change the fnmode parameter for the hid_apple kernel module.
Here's some documentation on it, but a quick summary can be found below: