A 9900k is a major bottleneck when you have a 3090, almost any CPU is in this game. My 2080 is often still too fast for the 9900k at 4K.
Try ultra setting, everything to the max and turn up render scaling to give the 3090 something to do.
Turn both the terrain level of detail and objects level of detail down to 100. Turn down AI traffic too. These are CPU heavy settings.
You want to be GPU bottlenecked as much as possible. The 3080/3090 need the fastest CPU’s in this game (singlethreaded speed, not more cores.).
Then watch the performance tab in task manager. Your GPU should run at (or near) 100%, if it doesn’t your CPU is still too slow. You will probably never reach 100% GPU usage with this CPU.