You're right, there is a negativity bias in reviews. That's mostly because people don't want to spend their free time and effort on making your product better.

Hire people specialised in Q&A, send someone over to big customers to observe how people use your product and ask/pay a customer to interview a random selection of the people using the software.

Or just stalk your customers. It doesn't require human interaction, it's cheap, and probably not illegal enough to actually get fined. Who cares about the actual opinions of your customers when you can just interpret some carefully selected dashboards, right?