v2 Client unable to connect to Desktop App

Ah, that's true. Turns out I installed 1password somewhat incorrectly. I only added _1password and _1password-gui to environment.systemPackages, but all that did was add their binaries to my path. The correct way to install them, with the ability to use the gui integration, was to enable them in the nixos configuration like so:
nix
programs = {
_1password.enable = true;
_1password-gui = {
enable = true;
polkitPolicyOwners = [ "my-user-name" ];
};
};

In hindsight, the documentation https://search.nixos.org/options?&query=_1password even mentions my specific issue, but I suppose I did not read it carefully enough. But it all works now at least. Thank you for the quick response!