What benefit does switching to postgresql bring? When mongodb stops being maintained, I'll consider postgres, but until then, I now only see downsides.

Lot's of useful features?

You can join data when needed, proper transactions, schemas where you need schemas.

For example you start with a basic (key, json) table. Once you begin to stabilise your schema, add columns generated from your json objects, now you can get proper joins, indexes, validations, on said columns.