If this setup could be combined with an easy way to customize things, like ublue (https://ublue.it/) lets you customzie fedora silverblue using Dockerfile configs, I'd adopt it in a heartbeat.

I’d think configuration.nix would be enough?

I have yet to find a non customisable nixos component. It'd a source system that's excellent at building from source so there's always a way.

Sorry, I was unclear, by easy I meant simple. Dockerfiles are simple. I've yet to find an equivalently simple way with Nix.

I haven't tried it but I would suggest looking here for some inspiration https://nixos.wiki/wiki/NixOS_Containers

What does the config for doing that look like?

They are likely talking about using https://github.com/nix-community/impermanence, which is inspired by ideas in https://grahamc.com/blog/erase-your-darlings/

Edit: comments on the latter at https://news.ycombinator.com/item?id=22856199

That's quite fancy, I use use a tmpfs mount for root and a few bind mounts for what I need to persist.

https://news.ycombinator.com/item?id=35390782

I posted a snippet before if you feel like checking my history Edit https://news.ycombinator.com/item?id=35390782

thank you!