I noticed this guide recommends compress-force=zstd, which sets the ZSTD compression to level 3. There’s a BTRFS benchmark and ZFS benchmark of the ZSTD levels which can give a rough idea of how ZSTD performs for transparent filesystem compression. Note that almost all of ZSTD’s compression gains happen starting at level 1, and levels after that have very minor improvements.

Also keep in mind that ZSTD levels only affect how long it takes to write new data to the filesystem. ZSTD is somewhat unique as a compression algorithm in that as you increase compression effort, the decompression effort stays the same. You could compress everything with level 23 and it will decompress just as fast as level 1 (~generally). Setting higher ZSTD levels could arguably make more sense for a gaming drive because the data is usually write-once, read-many. I don’t know at what level the Steam Deck CPU will limiting your I/O though.

BTRFS compression is enabled per-file, so you can change ZSTD levels at any time and old data will still be compressed with your previous algorithm. To recompress using a new level, change your /etc/fstab/ ZSTD level and remount the partition, then run a defrag to poke the data into recompression.