Slowroll does sound like a great model for versioning and patching software.
But for the actual package management I dont want to use anything but rpm-ostree. The immutable OpenSUSE variants are a joke and dont offer any real benefits over Tumbleweed to my knowledge (after having researched them).
Fedora may offer that “more stable package set” when sticking to the old release, currently F39. Not its still less seemless.