I don’t think auto-combining similar named communities is a viable solution, except in the case of users doing it themselves (e.g. multireddits or whatever).

Different communities, even with the “same name” (technically not possible because the @domain is part of the name), will have different vibes based on who participates, who moderates, what instances they’re on, etc. Mashing all of those together would at minimum, be a bad user experience and at worst, invite tons of harassment from ‘troll’ communities.