It seems this firmwareLinuxNonfree package already includes all of the firmware from the same linux-firmware repository that I am cloning to install the bluetooth firmware from!
I would have thought that including this package in hardware.firmware = [ ... ] should do the trick, however doing so does not appear to make a difference 
It looks like enableAllFirmware provides a superset, yet enabling this does not appear to make a difference either.
I’ve noticed a couple more kernel configuration options in the Arch kernel config that might help? I’m going to rebuild the kernel with these to see if they make a difference.
BT_INTEL m
BT_HCIUART_INTEL y
BT_HCIUART_SERDEV y