It sounds like community pruning is the better solution here. Users don't need to find dead remote communities in their search results. If there are multiple active communities, that's not an issue, and there's no real reason to homogenize them behind lizard brain FOMO. If there's one active community and 6 dead ones, there's no reason for users to find any of the dead ones.
Forcibly merging communities that exist on completely different websites just because they run the same, or even just similar, software continues to scream "I want centralization".
See, this compulsion needs to be killed off. Because no, they absolutely do not have to.