> If I were to do it in PostgreSQL 14 natively without any other third party plugins, I can get help of Table Partitioning (say root server) + Tables on Foreign Servers (say leaf servers) through FDW. Parallel scans and JOINs are allowed. Also, PostgreSQL 15 (in beta now) will allow parallel commits to foreign server tables through FDW.

I wish you all the best in operating that at scale and providing a reliable general purpose PostgreSQL DBaaS (even if only internally) around it. :-) PostgreSQL is great, not going to argue otherwise -- every solution has tradeoffs and if the tradeoffs of one solution fit your usage best then go with it. If you need to scale out and manage a large cluster of PostgreSQL instances as if it was a single logical database then I'd recommend something that's in a similar space as Vitess and PlanetScale -- Citus and Citus on Azure respectively. At the hand waving level it's always easy to "do this, and that", but it's a whole other matter when you develop a general purpose production-grade product that anyone can use and operate at virtually infinite scale. :-)