You're exactly right that our schema could be better and that it's non-trivial to execute on a schema change, especially because we actually run a distributed Postgres cluster via Citus AND we use a special sharding method that we manage manually.

We actually just started working towards how we might do a schema change for the 1+ million shards in our cluster. Hopefully, we'll be able to write up some learnings on the schema change after its done. :)