I don't think donations can build a sustainable model atleast for relays. Neither paid relays. Most want to use it for free.

Ads or promoted posts is the solution.

There needs to be a business view to it. Someone needs to build out an ad supported version of relay where the users have to see some ads.

Maybe bitcoin companies can be the first advertisers to push this forward. By paying some relay owners to run ads.

Once big corporations sees that running relay actually makes them money, that's when all rush to build businesses on top of Nostr starts.