You might be interested in Serpent OS, which offers immutability but without reboots after each upgrade.
https://serpentos.com/
They just hit their first alpha release, but it has been under development for years already. They focus on rust-based tooling, so even their coreutils are the rust versions instead of GNU. I read the alpha announcement yesterday, and might give it a spin later next year.
So far I've been very happy with Kinoite. I upgrade the base system once a week, but everything is installed in my Arch based container, so updates are fast and do not require a reboot.
On my workstation I use the Aurora Linux, a spin of Kinoite with extra tools such as tailscale added to the base image. On that machine I haven't needed to use rpm-ostree at all.
https://getaurora.dev/