Can I stop Relay attaching notes to my tasks?

Yes. In Settings → Task apps → Linked notes settings, turn off Link notes to tasks. Your tasks then get pushed on their own, exactly as if they never had a note: nothing is fetched from your Supernote, nothing is attached, and no note link is added. Each app's own note settings stay as you left them but are switched off while this is, so turning it back on picks up where you were. You can still open and read your linked notes inside Relay — this only controls what gets sent out to your task apps. If the "can't reach your Supernote" message appears when you push, you can also turn it off right there with Stop attaching notes.