To make your life easier, I would recommend that the root btrfs subvolume (id=5) is only used to manage other subvolumes and snapshots. Don't store
a lot of data on the root subvolume and don't reference it in scripts etc.
This will make it a lot easier for you to make snapshots, do roll-backs etc.
For example, in fstab:
# Root of btrfs volume. Used for volume management.
# Do not reference /mnt/root-XYZ/ in scripts or software.
LABEL=XYZ /mnt/root-XYZ btrfs noauto,subvolid=5
# Software using data on this subvolume should reference this path:
# and NOT /mnt/root-XYZ/corsair/...
LABEL=XYZ /media/qkiz/Corsair btrfs noauto,subvol=corsair
(you would change XYZ to your actual btrfs label)
Do put this into effect, you could create a snapshot of your root subvolume, and call it corsair. Probably also make a read-only snapshot of corsair, for backup purposes. Then delete files from the root subvolume. For example:
btrfs subvol snap . corsair
btrfs subvol snap -r corsair corsair-YYMMDD
Check that corsair-YYMMDD/ contains your important data, then delete the data from your root subvolume:
rm -rf ./datafiles....
Finally, mount your new subvolume:
You could make your 'corsair' subvolume the default subvolume if you wanted, but that is not strictly necessary. It could be useful if this is a portable drive and you move it to another machine with a different fstab. In that way, the corsair subvolume would be mounted by default, and not the root subvolume.