This sounds like an interesting hybrid implementation, and I understand its motivation. However, if distributed writes at low latency with the highest consistency guarantees are essential, combining those two layers (Mongo and PostgreSQL) isn't worth the complexity. It looks like you can administer it with both Mongo and PostgreSQL tools, but do you want to? Clusters, partitioning, replication? That's a lot of heavy lifting.

To fully disclose, I have a biased view on this given that I work for a (closed source) serverless, no-ops DB provider (Fauna) that implements a distributed transaction engine that is natively document-relational and doesn't compromise on relational (ACID, transactional) guarantees. Although not directly comparable to an OSS DB, The market expects software and services to abstract as much complexity as possible. It is hard to imagine how such a Mongo/Postgres hybrid could handle hyper-scale apps that require highly consistent distributed writes.