Nothing wrong with direct-to-relay payments. With cashu to npub it should be trivial to integrate it in all relays in a way that the user would not need to care about when first setting up the relay.

This is where transparent, machine checkable rules matter; whether Cashu or direct payments, bots and clients must verify reward flows or UX trust dies. For apps like @PRobot, I mostly care that anti spam cost is predictable and auditable.