The moderation is a separate concern, but I fully agree on cross-posting over consolidation, with moderation limited to wherever the initial posts were created. As someone that was a mod on a popular sub (/r/soccer) I can say that when you deal with those levels of traffic you absolutely need hard rules to ensure that only high-quality posts make their way through - and should Lemmy ever go through the same spike the same things will happen. You cannot just let people upvote/downvote as a quality filter because that's how to end up with bots winning, vote brigades, and content that focuses on the same shit. Some mods are useless, but I can happily say that /r/soccer was modded very well, despite insane challenges like users (literally) stalking mods, regular death threats, and some unhinged attempts to spam posts.

I have no idea how you avoid those kinds of issues. I don't think there is a solution, because no social network has solved this at scale. In terms of Lemmy, though, consolidation is absolutely required because many communities are dead.