At that point it's easier to run your own instance, I guess? As it stands now, it's not trivial to scale neither the db storage (postgres) nor the backend (lemmy).

I don’t think that users’ donating parts of compute is a way to go, honestly. You’ll have to think about bad actors (such users will effectively be instance admins for some subset of data), and it might quickly deteriorate into the weird crypto world (aka let's use blockchain as a storage because no one can be trusted to really count the upvotes).

Unfortunately, it's a very tricky issue to solve. I'd say donating to your instance so that its operators have enough finances to support it is the way.