I believe you can achieve this by making a separate usda file of your entity: right click the entity, and choose "New Scene From Selection". Make sure you have 1 root transform. This root's origin will be at the center of the scene. Now you drag your model entity (a child of this root) to the spot you want. Then in the original file, your entity has become a reference to that new scene you made. But rotating it now should keep the origin as per your new scene.

As far as I understand the current implementation for RealityKit/ARKit, you cannot auto-anchor Window Groups / windows / volumes to detected surfaces. You can only anchor entities inside of an immersive space. (you can however use mixed or full immersion, but you cannot use the shared space) This means, any auto-anchoring by the app will eliminate the possibility to multi-task for the user.