It's a fascinating problem. Might be interesting to release finances and provide a kind of open accounting for running a relay. Or like why don't any of the client projects add a donation badge in their UIs?

I wasn't very impressed with any of the paid-relay flows when I tried a few out. It was work for no measurable benefit. Maybe a combo with merch?

Overall it just may be too early given the insecurity of DMs via NIP-04. DMs would be an awesome way to invoice if secure. To that end, NIP-44 security audit needs more attention.