The first question you must ask yourself is: do I really need a document store?

Because the answer is "no" in the overwhelmingly majority of cases, specially if your product is mature.

Or trust me, I'm aware. But inevitably I will be in a design meeting where they will want a non-sql alternative, and I'd be nice to know what I can suggest besides Mongo

It depends what you're using it for. Postgres is a very good all-around choice these days (compared to when the whole 'noSql' thing got started) and also supports document-based scenarios quite well via JSON/JSONB columns and its support for these datatypes in queries, updates, indexing etc. Sharding and replication can also be set up via fairly general mechanisms, as described in pgSQL documentation. (For instance, the FDW facility is often used to set up sharding, but it could also support e.g. aggregation.)

Note that there is no Jepsen test for those sharding/replication features.

As has been mentioned above PostgreSQL does not come out of the box with a supported, tested clustering solution.

Given that is a pretty popular part of MongoDB seems like an important thing for people to continuously fail to mention.