Kernel arguments are the primary method: https://wiki.archlinux.org/title/Zswap#Using_kernel_boot_par...

Snag: I had issues getting it to use zstd at boot. Not sure if it's a bug or some peculiarity with Debian. Ended up compiling my own kernel for other reasons, and was finally able to get zstd by default, but otherwise I'd have to make/add it to a startup script.