Why not just use Postgres, set a field as jsonb, done?

Why use Postgres just to store JSON? The entire tooling around Mongo is better for JSON Data.

Not to mention the LINQ driver....

And you can’t do updates on individual JSON fields in Postgres.

Yes you can - this is an article crated in 2017 https://www.idalko.com/crud-operations-postgres-jsonb/

From the article,

The problematic part with PostgresSQL JSONB is modifying a JSONB column content. In any case, we totally replace the old content by a new one. So, the update operations turn to the complex queries that can lose the content. You can avoid this by performing the full JSON reorganization before the persistent operation.

You still don’t have the richness of MongoQuery and the aggregation framework and the tooling around working with JSON data isn’t there. Let alone the native driver support.

Postgres is better than Mongo for all of Mongo’s use cases, and 100 others besides, why wouldn’t you use it?

Including the Mongo aggregation framework, tooling, driver support, in the case I mentioned LINQ support, various reporting packages, etc?

Due to the sensitive nature of the data and the need to keep it encrypted, breaking it down by columns into a schema we could query was a non goal.