I think you misidentify the problem, or at least miss one huge problem.
Sure, empty timeline may be one problem, but an even more fundamental problem is that of the complication of having to choose an instance in the first place. It seems to me that's the real problem this addresses in the onboarding process.
If users don't even sign up because of choice paralysis of having to pick an instance, then it doesn't even matter whether timelines are empty. The user won't get that far.
I'm not sure what better option there could be for addressing that than getting the user in and then encouraging them to switch to an instance they might enjoy better.
For example, sending them to a random instance doesn't seem like a good way forward either.