This is my favourite hack for the Steam Deck. By switching my SD cards and the internal SSD to btrfs, the space savings are unreal (easily halving used space). Every game gets its own prefix which means a crazy amount of file duplication.
Which tool did you end up using for btrfs? I tried out bees https://github.com/Zygo/bees but it is way too slow.
How established was the drive when you set up bees? In my experience you really want to do the initial pass without snapshots existing, but after that it's pretty smooth.
Quite established. Although having read about snapshots causing slowness, I actually deleted all my snapshots and didn't notice any improvement.