Nix had a huge learning curve for most folks, but it doesn’t suffer most of my complaints about control.
Ironically a full Ubuntu modular system made up of a bunch of snaps wouldn’t necessarily either. One of the cool things about snaps is that they can hold the kernel and other lower level things so you could build a “snap”-together immutable system out of various components.
Silverblue and its variants are a monolithic system image though.