It is only for new data.

For example, you would have to defragment your filesystem again with btrfs filesystem defragment -r -v -czstd /. Where zstd is an algorithm and /, a root path. With this command, the default compression level will be used, which is level 3.

Be careful, defragmenting the btrfs file system will/can duplicate the data.

As for a mount point, if you decided to use zstd algorithm with level 1 compression, just add the compress=zstd:1 or compress-force=zstd:1 to the mount options (fstab or while mounting manually)

It generates initramfs images faster. Also the generated images are much smaller (30-40MB for fallback images).

The only problem is that the tpm2 unlocking doesn’t work currently.

No, but according to this Phoronix article, they will fix the RAID56 issues soon:

The support for RAID56 is in development and will eventually fix the problems with the current implementation. This is a backward incompatible feature and has to be enabled at mkfs time.