I recently got a Bluetooth keyboard and it was fine but one problem I find is that I need a wired keyboard to enter my password on the sddm login.
To use your keyboard on startup you just need to start Bluetooth before you can log in, to do that you need to open /etc/bluetooth/main.conf on any text editor and uncomment and change the AutoEnable option from false to true.
$ sudo nano /etc/bluetooth/main.conf
use ctrl + w to search for a Autoenable in nano
go to this line and change the autoenable option to true
/# AutoEnable defines option to enable all controllers when they are found.
/# This includes adapters present on start as well as adapters that are plugged
/# in later on. Defaults to ‘false’.