Excluding conversations about MongoDB compatibility, PG16 added bidirectional replication for multiple writers and there are Postgres-compatible options out there for a distributed database including Citus, EDB Postgres Distributed, Yugabyte, CockroachDB, Aurora Limitless, etc.
The choices require some nuance to figure out a best fit, but then again so does any MongoDB installation (despite the marketing hype to the contrary as there are no free lunches).
You might be surprised how far most folks can typically scale with just read replica(s) on a reasonably sized writer. Add in bidirectional replication for multiple writers, and you can go even further. Beyond that, even vanilla Postgres can do it, but you'll need to do some combinations of partitioning and foreign tables.
The choices require some nuance to figure out a best fit, but then again so does any MongoDB installation (despite the marketing hype to the contrary as there are no free lunches).
You might be surprised how far most folks can typically scale with just read replica(s) on a reasonably sized writer. Add in bidirectional replication for multiple writers, and you can go even further. Beyond that, even vanilla Postgres can do it, but you'll need to do some combinations of partitioning and foreign tables.