I would recommend against modeling too many relations in a document DB. It can work, but it gets bogged down very quickly for the reasons you've stated.

My off-the-cuff suggestion is that document databases are not built to model normalized relational data. If you try to do so, you bring a whole new level of pain upon yourself. It is do-able, but it is hard and annoying.

I know this from extensive personal experience dealing with a large, highly relational Mongo database.

I am very glad you found a solution within Postgres that works for you, and your mapping of document to row and collection to table is very apt!

If possible, would you care to tell me what the size of said documents (rows) and collections (tables) are in your solution? I am curious if in another life, we might have built our tech stack on Postgres instead of MongoDB and been much happier for doing so.