This sounds strange to me. I have a lower spec GPU (RTX3080 vs your RTX 3090 Ti) and processor (Ryzen 7 3700X vs your 5800x3D) and I am running 4K with virtually everything on Ultra and getting acceptable framerates. By acceptable I mean 40 - 50fps flying very low over forests for instance and dropping down to 30s in some heavy custom airports like the FSimStudios CYVR Vancouver. Some examples:
Over New York, showing 41fps. And note that the image is downsized from 4K to HD due to the forum’s software:
RTMM scenery at Zeballos CAA5 in BC, Canada: had 1round 45fps here
I have turned off things like Multiplayer and motion blur etc., but for the rest it is pretty much all as per the default Ultra settings. So it does sound strange that you are struggling with your powerful rig. If you want I can do screenshots of my settings