From the docs I think Limmat is much more minimal. It doesn't have a merge queue or anything, "jobs" are just commands that run in a worktree.
I would be interested to try SelfCI coz I have actually gone back and forth on whether I want that merge queue feature in Limmat. Sometimes I think for that feature I no longer want it to be a local tool but actually I just want a "proper CI system" that isn't a huge headache to configure.