Is there a way that users could cohost an instance? If there isn’t, I think that would be a great feature to implement. Having a situation where instances slow down or even occasionally disconnect will be hard on any community. I feel like it could eventually cause people to prefer using instances owned by large and well funded groups instead of passionate users.
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.