@egal There’s no issue with using Wyng on Btrfs except for a matter of initial setup. Wyng requires that the –local storage path on Btrfs end with a subvolume. The util handles the local parameter automatically for you when backing up, based on where Qubes is storing volumes, but it can’t do the initial step of creating a subvolume.
There is a script that can convert the Qubes storage dir to a subvolume. If you used the default Qubes Btrfs setup then you would run it like this:
qvm-shutdown --all --force
sudo bash convert-dir-to-subvol.sh /var/lib/qubes
If you didn’t use the default Qubes Btrfs setup you can find the dir path with the qvm-pool command.