How better? Better machines or what?

How about some initial sats payment that is automatically reimbursed if the relay operator does not decide to slash it? That would allow for much higher prepayment but come at low cost for honest users

I think that was the so Sphinx chat model. You give up sats to join a group and send messages and if the chat room owner decided it was spam they could keep the sats.

which is a perfectly viable model I think. It would even allow to make it free for honest users and very costly for dishonest ones. There's always the possibility of rugs by the relay operators, but that's the risk a user is taking.