I will tell you the truth but you won't like it. Your CPU is holding your 3080 back. It's not fast enough to drive it to the point where your FPS will get better. The trust is that MSFS is very cpu intensive but add the 3080 on top of it and yes Mainthread limited is what you will see all day.
That being said. Asobo KJFK is pretty heavy when photogrammettry is active. I can get around 35fps while at the terminal but that's with an AMD 5800X3D which is the kings of FPS in MSFS. If I go into an airport like CYVR, I get 60fps or more with the same graphical settings. If you want more FPS in the DC-6, enable dev mop and check NanoVG XML option. That will give you another 10-15fps.
This is an example of what I am getting in the Maddog MD-82 with my setup. This is all because my CPU is the best on the market for MSFS. If you really want to take advantage of the great GPU you have, Either buy an Intel 12900KS or switch to an AMD 5800X3D. You will get more FPS on AMD than Intel.