I think the vision is that the overflow table would never be very large
because it can be vacuumed very aggressively. It has only tuples that are busy
and will need vacuuming as soon as a transaction ends. Unlike the main table
which is mostly tuples that don't need vacuuming.
Thats right. vacuum if it gets a chance to work on the overflow relation seems to be doing a decent job in our runs. If autovacuum/vacuum gets to run optimally, the FSM information generated for the overflow relations will be able to serve a lot of new tuple requests avoiding undue/large bloat in the overflow relations.