

The system will always boot to a writable snapshot, unless you chose an old one (which becomes read only) If you have the 835 as a single snapshot that is probably something you made a single snapshot of maybe? as they are usually in pairs.
Unless something went really odd.
Lookup tools to purge old btrfs data, but first try manually making a new snapshot. And see if it will boot to the new one on its own. If it can you can probably delete the old snapshot pairs. The singles are often a large install like original install that served as the basis for subsequent incremental changes. There could be a lot of old data it is hanging onto.
Also look at the btrfs scripts to see when they’d run, you can run the commands manually to try to cleanup and purge data




I forget the commands but there is something like btrfs scrub or something that will prune or purge unnecessary data. But also is your /home on the same snapshot fs. Like when you do a df -h do you see same root fullness as home fullness? A full home also fills root, if they are same btrfs volume.
I know you are drive mounting into home, I assume after the fact and not part of initial lvm setup. You could try unmounting and sew homes true size compared to root