If the goal is to talk about your interests with strangers (and not pull in your whole family and political and journalistic class), some minimal federation friction could be beneficial actually, as a soft filter. Maybe it's worth it to have some common pre-Eternal September spaces on the Internet, even it means not building unicorn empires.

The latter seems to be a mindset people aren't able to shake off for some reason, applicable to the given case or not. If a person can write and read two sorta coherent paragraphs of text for a internet forum, they are already on the level of being willing to learn and a little curious, above what the tech giants expect from their model user. There are enough of such people to have interesting and diverse discussions. Don't tell me Discord is easy to figure out. I'm not saying the devs should be working to make the experience easier, but it starts to be one of those topics where the discussion is meme-based.

You can tell search engine robots not to index your copies of different forums. I don't think they are viewable by default anyway? Avoiding small instances to avoid outages sounds like 4D strats to most users, who might prefer cosiness, personal connections and camaraderie, like on Minecraft servers, say.