As email in profile is an optional feature on our instance about 50% of the userbase opted to not share one so we would only reach some people. But yes, getting spam filtered would be another problem of mass-mailing.
But I found a way to put up a simple html error page and will add this later to inform our members and link to this thread.
Not necessarily, no. We aim to preserve users, communities and posts/comments. Image uploads might get lost though.
Such an in-place migration will need extensive database operations and likely some support by the Piefed developer (to add support for bcrypt hashed passwords), but we are hopeful to make it happen and maybe this will result in a database migration script other Lemmy instances could also use.
If this turns out to be infeasible, we will stay with Lemmy rather than reset everything.