Before you do anything, make a backup of the db.
Grab an interactive sqlite tool.
Stop the nix-daemon and its socket. We don’t want it accessing the db.
Then you can take a look at the DB in sqlite and try to fix its issues manually. Perhaps it’s just a few malformed entries that you can simply fix manually with a few SQL queries.
Otherwise, you can always just re-install NixOS via nixos-install (obviously keep all your data and only wipe /nix). If you’re on btrfs, this could even be done on a live system.