I’ve had a similar idea before for the join sites where they asks users a few questions about their interests/preferences and then redirect them to an instance based on that information. And when they sign up, it could also automatically subscribe them to the relevant hashtags/communities. Instance admins could be prompted when setting up the software if they want the site to send users to their instance (and how many they’re willing to take); this could work like a firehose distribution system to help avoid overwhelming infrastructure during exodus waves.
The questions should be fine for most people so long as they don’t go overboard with it. 2-3 should work. Select interests from a list (and have an option to fill some in), select moderation preferences from a list, and select a language/region from a list. There could even be a button that just selects an instance randomly for people who can’t be bothered (this can use the browser language to avoid sending people to instances in languages that they don’t speak).