Btrfs is cool, nixos is cool. Together they are pretty good.
You can use snapshots in addition to nix's links to ensure
reproducibility of state - stuff that isn't managed by nix. For example
var, keys, out-of-nix configs like hydra, keys. But don't
forget to encrypt it all, or your secrets will end up in plaintext on
your NAS.
The main question is what to separate. Snapshots allow you to inspect entire filesystem at some point in time - both state and nix's links. Hovewer the two have different requirements and you might want to keep their separation as high as possible.
Consider the following distribution of mounts.
/boot - different fs mounted./nix - a subvolume, no snapshots./var - a subvolume, frequent snapshots - since FHS
suggests to keep state here, e.g. logs./home - a subvolume, snapshots depend on usecase./ - also a subvolume, infrequent snapshots - mainly for
state in etc.All these subvolumes should be created on a separate fs, which itself isn't mounted.
Hard to do on a running? probably easier on a cold drive and
straigt-forward when installing. The easies approach here is to backup
your state, i.e. /, than do a full reinstall and lift
out-of-nix state from the backup.