I think that a phase of low community engagement is the best time for a migration. Much easier than with a large community, since only the Grin diehards are left that would follow anywhere.
While most Big Tech platforms try to force you to install their app and provide personal information, most of the fediverse apps are available as webapps and support mutliple other open source clients. Create account and you’re in. Much lower friction in that regard. While there is more friction usually with the UX, because they are built on much less money, but honestly, most of them are pretty good.
I’d agree for the community communities, but I disagree when it comes to the “official” communities, like the dev channel or governance. These should be as open as possible.
Keybase makes it hard to create a backup. We are at risk to lose the early history. I think this would be easier with a more open platform.