For grub-btrfs.path, i wrote this:
Mount your subvolume which contains snapshots to / .snapshots
which means :
You need to create the .snapshots folder.
and you need to mount or create snapshots in it.
Why Mount your subvolume which contains snapshots ?
Because my layout of btrfs subvolume is :
@racine (my subvolume containing linux root system)
@snapshots (my subvolume containing my manuals snapshots)
etc…
Your is :
@
@home
etc…
Timeshift creates its working folder at the root of the btrfs system top level 5.
You can check it like this:
mount -o subvolid=5 /dev/sda1 /mnt
This will mount the root of the btrfs filesystem in /mnt.
Remember to modify /dev/sda1 if it isn’t your btrfs system partition, and /mnt if you want to mount it elsewhere.
You can see your subvolumes and timeshift working folder there.
for example:
@
@home
timeshift
etc…
Then timeshift mounts this folder to/run/timeshift...
I don’t use timeshift, but if you want to make grub-btrfs.path work, the easiest way would be to modify the service like this:
[Unit]
Description=Monitors for new snapshots
[Path]
PathModified=/run/timeshift/backup/timeshift-btrfs/snapshots
[Install]
WantedBy=multi-user.target
Or something close.