Well since my last post, I went and upgraded to a 4090 myself 
so I can give you a better idea of what to expect with a 5800X3D paired with a 4090.
The screenshot below gives you an idea of how a 5800X3D and 4090 can run with an addon aircraft (Kodiak 100), albeit at a remote location in the Solomon Islands, and with a final render resolution the same as what you were showing in your last screenshot post. As you can see, plenty of VRAM for MSFS to play with and frametimes for the GPU are fast, as you would expect for a 4090, but they are also fast for the CPU in mainthread, like 3x faster than yours for the same CPU.
If you want I can load up a flight somewhere more complex of your choosing in the FBW (I don’t have the Fenix sorry) or the A310 (which is heavy on CPU like the Fenix is) and see what I get. I can also try your exact settings if you want, which you would obviously have to let me know. For reference, my settings, which are the same as I was using with my 4080 except custom render scale was 150%, are:
