I think we might be talking past each other. I understand that not every instance is federated with every other instance, and that there are differences between the instances.
Each instance is one piece in a larger mosaic – but looking for niche interests inside one particular instance is a bad venn diagram. The choice of instance should be of secondary consideration when it comes to niche interests. I barely care from what instance someone hails if they’re giving me cooking advice. This is why we have federation in the first place – it just needs to be a smoother experience.
I don’t see why that is necessary, as different posts could be hosted on different instances. Either way, the choice of instance is of little concern for me as an end user, so it should be de-emphasized.