There are other options, but I think you do have the right drift overall, 
…
Fly likes NATS a lot, but it’s definitely not the only messaging system. And a person could also consider non-Postgres databases that are designed around eventually consistent, multi-master writes (as the documentation mentions in passing).
I go the Elixir/Erlang route myself (+ LiteFS primary-swapping hijinks, maybe, for “pickle jar” cases), but @poundifdef is promoting SQS on Fly.io lately, if I understand correctly.
He might be able to give some quick intuitions about whether that messaging subsystem would be easier in this particular context…