I've been using zstd compression on btrfs for a while now and it's excellent, most of my stuff is already compressed (movies, music) but my home directory (which is mostly comprised of text files) has shrunk greatly.
The next GRUB release (grub-2.04) includes my patch to add support for zstd compressed BtrFS filesystems, which should solve one of the major pain points of Zstd BtrFS compression.
A temporary work around in the meantime, I've used `chattr +C` on the directories I want to be exempt from zstd compression, so that grub can read those files.