Unlock over SSH terminates the connection after unlocking the data volume, so it doesn't even attempt to start the shell until you reconnect after it's fully booted up.

FWIW you can fix the shell issue by wrapping your shell in a shim that essentially runs wait4path on the nix store before exec'ing your real shell. I set up my environment to install that shim binary directly onto the data volume at a known path so it can be used as my login shell.

Depending on the timeouts involved, I imagined it might still happen if you had automatic retry.

And thanks for the pointer, I actually have the same fix in my config with the nice benefit of only adding a single non-changing entry to /etc/shells. It might be worth up streaming something like this to nix-darwin, so we don't all go implement essentially the same fix.