I agree that underlining is a clear, and fairly clean, way of distinguishing the menus - and I agree that distinguishing the menus is worthwhile. The darkened background boxes on the 42s and 32s ii were less visually distracting, more pleasing to the eye... to my eyes. But they do cost an extra colour. Maybe look into if a really thin outline box is an option?
I'm fairly neutral on busy faceplate vs deep or many menus when my memory is not enough. Whichever it is, I want the labelling to be as informative as possible, and grouping/structure (especially for menus) to be as logical as possible, as these things helps the memory and reduces the hunting. IMHO.
However, my opinion may be disregarded, as I'm not a 43s user.