Do you have any pointers on how to get this setup working

Assume I have worked with rpm based distros in the distant past (last time at work 10 or so years ago) and with Ubuntu and derived distros a bit more.

I doubt that.

Does Opensuse have an integrated declarative configuration management system?

How would I apply a patch to a program on Opensuse and build that program the same way as the distro-provided package was (i.e. into a rpm that I can then install)? What about changing compiler flags?

How do I get an isolated development environment going with Opensuse, i.e. one that does not pollute the list of system-wide installed packages?

How can I check if a package provided in the Opensuse repositories was not tampered with, i.e. the provided build instructions actually produce this package? Are there even build instructions for the rpm's provided?

There is a lot more that nix and NixOS can do, this is just part of what came to mind on what I would assume is mostly impossible on "traditional" distros.

Also, I use a lot of software that likes being funny, do unspecified system-wide modifications while being outside of the package manager system therefore break it in non-reproducible ways down the line.

I currently went and wandered into "I just want things to not break by default" [1] territory, and I use the semi-immutable systems of Fedora Silverblue/Sericea.

I saw openSUSE Aeon/Kalpa, and liked the fact that it also has SELinux enabled, but it looks like the snapshot system interacts in bad ways with the overlay systems.

[1] Things that want to install garbage even as root in random places will get either denied by SELinux or reverted once a new system update layer happens.