I think that approach is needlessly complicated and would confuse people more than help them. There should be a way for individual users to merge the feeds from multiple communities in a multilemmy if they specifically choose to.
But I don't see any way to create such multireddits/multilemmys automatically because there is no objective categorization. Everyone is going to have a different opinion about which communities fit together and which ones are similar but different enough to keep separate. Instead of forcing a generalist solution that makes everyone unhappy, just give people the tools to build their own solution that works best for them.