First, welcome to the forum, and sorry, I just saw this post today.
When I had the 4090 on my i9-9900k, which meant pcie-3. And DDR4 RAM, I could max everything except terrain LOD, which I left at 200. And I was getting a nice 4K60fps and I could get more, but I called it there so I could record at 4K60 too for YouTube.
Now I have i9-13900k ( PCIe-4 and DDR5 RAM) and with everything maxed, and terrain LOD at 300, I get between 100 to 120FPS and 4K. and while recording with OBS at 4K60 it comes down to 90-110fps. Even at the highest density airports like IniBuilds EGLL, KJFK and KLAX. They don’t affect the frames at all.
So I think you should max everything and keep the terrain LOD at 200. Just choose DX12, and HAGS on, then TAA and Frame Generation on.
In order to have Frame Gen on, you must be on DX12 and have HAGS turned on. Will double your FPS or more.
One more thing, I only use NVMe SSDs, and the high speed ones. If you can, get the Samsung 990 pro or 980 pro. I have 5 x 2TB Samsung 990 pros, and one of them is dedicated to MSFS.
Also, I see that your monitor is 165Hz, and freesync. I don’t know if it is G-Sync compatible. If it is, disable V-Sync. I’ll show you my NV Control panel settings. Standby…