I have a very similar system as you do (6700k with a Strix 1080 OC) and my GPU can practically take off when I am in the MSFS menu.
So the menu gpu usage being really high is pretty well known ( I think). You might have to frame limit/cap and turn vsync on in nvidia controller. I cap mine at 50 fps just because that’s what I used for P3D and my eyes have no issues with it. (it’s a personal preference)
Basically from what I’ve gathered from the forums is that in the menu, the game just draws max performance, i.e. max fps, possible of your system even if your monitor is limited to 60fps. So think of having 200+ fps in CSGO but your monitor is still at 60hz. Asobo unfortunately did not make it so it doesn’t draw max power/performance through its coding so we are left to cap the menu performance on our own.
As for GPU usage while flying, I have no clue why you’d have such low fps and low usage. My Strix 1080 gets up to 70% (on average) and can draw 30+ fps normally (in flight, 8k ft or above in altitude). Only exception is when I am in a traffic heavy airport like LAX, only then I dip to average 20 fps. I have my settings generally in high-ultra settings. The ones in ultra the texture settings, draw distance (manually to LOD = 3.0 in .cfg file), and render scale to 130 (so buttons doesn’t look blurry on 1080p displays).