would be cool to have that also in planning mode (then compared to the altitudes I have planned on the legs), since it helps a lot to plan my way through valleys when mountain flying (right now I always have to use the flight or sim mode)

Great idea, but I cannot imagine an algorithm that can handle a complete route when planning legs with different altitudes.

Proposal: tapping on a leg in planning mode switches to terrain safe display. Planned altitude for that leg is the reference altitude for the terrain safe display.