So there are more than one things to be adjusted :-).

Thank you, I'll look at the data with a fresh head later, but it seems
to me that we'd need to have GRUB fallback logic for the root devices
when RAID setups are detected.  I'll read on what GRUB has in store for
this kind of thing when I have a chance.

Your Debian GRUB config also has me wondering about the 'btrfs' modules,
and the others than Guix System is not using.

Thank you!

Maxim