Source (link to git-repo or to original if based on someone elses unmodified work): https://github.com/shvchk/dolphin-compress-to-squashfs
SquashFS can be easily mounted, so it's often more convenient than usual archives for backups and other purposes.
No root required (but there is an option to use root).
When using zstd compression, you can choose compression level.
See also SquashFS mounter / unmounter (no root required, too): https://pling.com/p/1410382
Installation: place the file in ~/.local/share/kservices5/ServiceMenus