My (in)experience with Mongo leaves me with a certain existential dread that I just can't shake. Also for that sort of thing these days I'd be more likely to use ElasticSearch than anything (this absolutely terrifies me actually, but I'm seeing companies do this) and eat the extra infrastructure expense. Serves the same function and gives me insanely powerful tools to process the data without having to write any code.

In my planning for problem domain changing, personally I'm going to be writing a (relational) schema for the new data model anyway, which will be Postgres, and playing my event store over it and doing one-way writes to it going forward anyway. I'd rather reduce the mental maintenance overhead of two different databases to one.