Honestly I think having a default one is necessary for users to put their trust in the service

As I said:

Getting a list of some instances (that are not as overcrowded) and assigning one at random by default would be the best way to go about it.

People want to have their account on the instance all their friends have it on so that they’d never be alone in losing their account or the service being offline for them

That... doesn't make sense.