Another option would be to take the same strategy as non-profit/community run radio stations have been doing for decades: fund drives.
It's really just a form of advertising except the only advetiser is the station itself. So if a relay can inject ads, surely they could also inject messages asking for support from the users.
I've seen a mastodon server that sells swag to help with the costs, which is basically the same thing. However they also have closed registration.
Is there something similar to the "closed registration" model for nostr relays? Allow anyone to read posts from a relay but only authenticated users can post?