Your complaint is that SQL databases aren't message brokers? SQL servers have been abused as message brokers for decades. It's not the use case they're designed

Your complaint is that SQL databases aren't message brokers? SQL servers have been abused as message brokers for decades. It's not the use case they're designed for, and while a plugin that integrates this type of functionality into PgSQL would be interesting, there's no reason to expect them to assume that problem space. If you want the database to handle the incoming message streams, then set your consumers to call a stored procedure. That should alleviate your concern with conflicting data models.