Regarding RAM: Check out this thread. Forum members share their own opinions about this topic.
https://forums.flightsimulator.com/t/is-it-worth-using-96gb-of-ram-instead-of-64gb/704261
As for X3D: The graphs in the image below clearly show that the sim uses the X3D cores. Granted, my test was @ 1080p with CPU-bottlenecked settings. And it doesn’t matter all that much to me with my outdated GPU @ 4K. But if I had a 5090 (or even a 4090) I’d bet that I’d see a much bigger advantage with my X3D vCores. Notice the dramatic difference in FPS and latency when I assigned the sim to vCores only, vs. non-Vcores, or even all-cores. There’s no argument that the sim utilizes the X3D chiplet.
Like I said, the dual-chiplet design of my 7950X3D needs some TLC (and Process Lasso) to get optimized. The 9900X wouldn’t need as much work, and should be just fine. But you said, “No compromises” and I believe an X3D CPU would help you reach that goal.