I’m not sure about seeding. It’s a very powerful tool, but it can backfire very badly.
Having a ton of imported content, most of which doesn’t have an owner (because the owner didn’t move from SE, in addition to all the owners who had already left SE before), is only viable if there are enough to do some curation and to answer the new questions. Curation of old content includes moderate edits, retag as needed, update broken links and whatever else needs to be updated, etc.
There are plenty of sites around that just grab SE content wholesale and display it, plus ads. Some of them are read-only, but some of them are actual Q&A sites. Even the actual Q&A sites are worthless because no one is around to curate them. Occasionally a poor soul asks a question there, and no one answers and it’s lost in the mass.
I wonder if it would be better to migrate content on explicit request rather than wholesale. Perhaps let a user import all of the threads where they participated as an asker or answerer, or a subset. And maybe have a privilege that lets users import other threads (tag-based, maybe?), but that part wouldn’t be MVP.
Would new communities start empty (private beta) or start with some imported content? Starting with imported content is only viable if the community policies are close enough.