That is just not true that “most of the time the sim is main thread limited”. I play on 5800X and rtx3080 in 1600p on ultrawide and the sim is only main thread limited when there is a lot of photogrammetry (only large cities with PG). Otherwise it is ALWAYS GPU limited for me. Same in 4k and I suspect in 1440 as well.
Why flight sim is not featured in many benchmarks is because it is really difficult to benchmark. There is no benchmark mode and the game is really inconsistent and dependent on the plane choice and location. Reviewers usually choose to benchmark JFK landing but other reviewers choose another challenge which makes results almost impossible to compare because the fps range from 25-50fps average depending on the landing challenge.
If you plan to play in 4k than the latest processor is not THAT important, it may give you a few fps over large cities. However, the latest GPU makes a big difference.