Fedora IoT is an easy step to this as well. It's really "Silverblue server", rpm-ostree based but without making the jump to ignition files right away.
at least to me the declarative setup of ignition was the selling point, not the rpmostree (altho it is nicer than dnf/old school rpm). Another cool thing about fedora-based is that it seems much more bleeding edge in terms of kernel version on other os packages than chromiumos which is what I want for immutable infrastructure
For more on immutable / image-based Linux you might want to check out the UAPI group, a recently founded cross-distro interest group: https://uapi-group.org/
We have contributing members from many FOSS projects gravitating around image-based Linux, including quite a few distros.