Thank you for the reply!

What do you mean by declarative system configuration? that thing that nixos does that you set it up thru its config file?

What you refer to in NixOS is indeed its solution to offer declarative system configuration. But the other two mature immutable distros, i.e. Fedora Atomic and Guix System, have their own solutions. Though, Guix System's solution is a lot more reminiscent of NixOS'. While Fedora Atomic leans on 'the ways' established for OCI (and hence containerfile(s) etc). Even less mature immutable distros, i.e. blendOS and Vanilla OS, have put considerable effort into the works for managing their systems declaratively.

I’ve also kept several month old btrfs snapshots on my system and I don’t see a problem with it, they only add like 3 GIB of storage each when they are that old.

My argument here is mostly just "No occupied storage on device is better than some occupied storage on device.". But yeah, its significance is definitely up-to-debate. Perhaps I should have relied more on the built-in aspect; from the mainstream independent and/or highly popular traditional distros only (Garuda,) Linux Mint(, Manjaro, Nobara) and openSUSE Tumbleweed come with built-in rollback/snapshot functionality. But, regardless, the rollback/snapshot part of the equation is definitely the least special (if at all).

Also I’m not sure what you meant by increased security? Is it more secure simply because you can’t edit the root filesystem?

It's indeed related to how some parts of the system are read-only during runtime (under normal circumstances). Hence, some types of attacks are circumvented from the get-go. This, by itself, doesn't warrant the use of an immutable distro over a traditional one; even if the user is security conscious. However, if said user already intends to use a distro that takes security seriously (i.e. Fedora or openSUSE) for the sake of security (or at least it plays some role in their decision-making), then they might as well prefer their atomic counterparts. But yeah, for actual security, one should probs rely on Qubes OS instead. Though, atomic distros have given us the likes of secureblue; which may be the most secure Linux system for general-use we got (besides Qubes OS, if we even count that as Linux). The only other contender is Kicksecure.