Wrote this; the MultiXact system in Postgres is really pretty complicated but not something you come across _that_ often (unless you're running workloads like this) - it's worth understanding exactly the mechanics of what's happening when you do hit scaling issues like this and if something like the leasing we do here is possible.
I enjoyed this being your final post before you left. Quite the swan song.
Do ticks ever outlive the lease? How is that handled?