If Nix is too much but you still want an immutable distro, look into Fedora Silverblue (or Kinoite). It offers some of the advantages of Nix in a more approachable fashion.
Its the first distro I've used in 20 years that doesn't feel like a stiff wind could cause the whole thing to collapse and catch fire in an unrecoverable way, while still allowing me to run any software I want instead of just what is in some limited curated repo. And unlike Nix, I didn't need to learn a new language to use it.
toolbox create new_dev_env_or_whatever
toolbox enter new_dev_env_or_whatever
dnf install all_the_things
I have my issues with both Flatpak and Toolbox as far as how they're put together and work, but overall I find it to be a vast improvement on the status-quo way of doing things. YMMV, of course.
The limitations are:
1) Each invocation of the package manager (install/upgrade/uninstall, the usual stuff) results in a new OS 'commit' and requires a reboot to switch to it.
This is the entire point of the distro (random .rpm broke your system? just rollback), and is why Silverblue users generally only use DNF packages for low-level tools and run regular applications as flatpaks, appimages, or podmans.
2) Occasionally you'll run into a package that wants to write to a folder that Silverblue made read-only and version-controlled, and breaks. In that case your only options are to run the package inside a toolbox (a special Podman container that comes with Silverblue for these scenarios), or patching the package.
https://gitlab.com/ahayzen/silverblue-nix
NixOS people will prefer NixOS, but Silverblue seems like a nice complement to Nix if you need an FHS base system and want to retain some Nix-ish features like rollbacks and atomic upgrades.