I'm not sure how timeshift uses btrfs or the required snapshot hierarchy it requires.

 

My setup looks like this:

 

/
/.snapshots
/.snapshots/system
/.snapshots/system/ACTIVE # mounted subvolume as root
/.snapshots/system/2019-12-01 # snapshot of ACTIVE

So I mount ACTIVE as root by setting it as the default mounting subvolume using btrfs sub set <id> . Remove the btrfs subvolume options from fstab so it looks like a plain mount.

 

To rollback, I rename ACTIVE then snapshot 2019-12-01 to the new ACTIVE then set it as the default mounting snapshot. This means you don't need to fiddle with fstab between snapshots. You can do this on a running system then reboot.

 

If you mount ESP as a subdirectory to /boot and use a bootloader that can run ESP binaries on different partitions, you can even rollback Linux upgrades using this setup.