Installed x2goclient-4.1.2.0-2018.06.22 and performed additional testing. It seems that if I specify a key in the "Use RSA/DSA key for ssh connection" the client doesn't attempt to use the ssh agent at all, only that specific key file. I had thought that it would still try the agent first, but it only tries the specified key file and then moves right to asking for the password, skipping the agent altogether.
Next I created a new id_ed25519.ppk file and loaded it with pagent. I leave "Use RSA/DSA key for ssh connection" blank. The client now immediately uses pagent for authentication and launches my session without requiring any further interaction.
OK to close, thanks! -Ross