With a subnet model, the main layer would have just a basic functionality as you say. Exact feature set is to be determined, there would probably be more than what you listed, related for instance to validator selection/rotation and stuff like that, maybe some governance related features, cross-subnet communication.
Catalog will not be a subnet, important distinction, it’s an app that has to live somewhere, like a subnet.
The fee model in relation to Catalog is not determined, has to be discussed and worked out and proposed.