This is a small subtask of:

• Store (eye) camera properties of 3D View in Blueprint #6825

3D spatial views decide their default camera pose heuristically based on a smoothed bounding box. This default pose is what is used on start & when resetting the camera.
The first step into configurable cameras is therefore to set the default position & direction for this default camera.

This is mostly orthogonal to all the other camera settings: it doesn't have any relation to how the camera is controlled (ego/orbit/other, follow-mode, movement/rotation sensitivity).
The accompanying ui should have two buttons for:

• setting the current camera dir/pos as the default
• resetting the camera to the default (same thing as double clicking in the view)

This was started in

• Adding DefaultCamera to Spatial3D view #8211

Proposing to use keep the simple look-at camera style here that was used in that draft:

• position
• target position
• optional up axis (sourced from View Coordinates, this part isn't in the linked draft yet)