Pretty much, unfortunately. It sucks, because in order for Nix to accomplish its vision, things have to be like this - I don’t really see a way around it.
I am amazed by what the Nix[OS] community has accomplished and give high respect to them for it, but I can’t do it. If the documentation (and procedures, eg Flakes) were a bit more structured I’d probably be a bit more willing to put more time into trying to figure it out but… that’s just not the case currently.
I have similar feelings about immutable distros, it is a very intriguing concept but every single time I’ve tried one out, I run into some issue that requires hacks to get around it. If I did end up using one long-term, it’d probably be something from Universal Blue because it seems fairly easy to just modify the image. However, it’s still a massive paradigm shift of getting used to making changes at build-time (of the image), rather than making changes to your system at runtime.
For now, I just do pretty much the same thing you do, important dotfiles go into git, and btrfs snapshots for “Uh oh, something broke and I need things to work right now” moments (which is thankfully quite rare).