When the G+ shutdown was announced (see earlier comment, I was one of the "Plexodus" organisers), the whole question of what to do about Communities migration hadn't even occurred to me. It wasn't until a community moderator happened to mention that they had a group with 400,000 subscribers and no idea at all to even begin that I started looking into this.

The first result was to create at least a framework for organising and planning: https://social.antefriguserat.de/index.php/Migrating_Google%...

It's a really basic thing. I've found no evidence that any such guide has ever existed online. I make no claims that what I came up with was the best, or even effective. It does seem to have been the first. Win, me.

We created a "short FAQ", formatted for posting to G+, which we posted again and again and again and again and again, to as many communities and spaces as we could. Because when you're trying to reach a mass audience (10ks to millions of users, 10ks to 8.1 million communities), mass repetition is necessary.

https://social.antefriguserat.de/index.php/G%2BMM_Short_FAQ

It's hard enough to motivate people to turn up at one place once every couple of years to mark a few spots on paper, when they all know that this happens regularly -- election turnouts tend around 30-40%. Getting mass compliance on a complicated, unexpected, and technical process (with social and legal entanglements) is far harder.

Many groups glide by on minimum maintenance for years. Whatever initial technical competence was required to set up the group be long-lost. At best, moderation skills are known (and even that is a long stretch). Migration is an entirely different skillset, one rarely exercised, and, as noted above, extraordinarily poorly supported.

TL;DR: your expectations are entirely unreasonable.