Hmm, running MSFS in 4K is not unrealistic at all. It might be if you have a low to mid grade PC, but with a high end PC such as a 5800X3D, it shouldn’t be a problem. Not unless there is something causing the bad performance.
With my setup and now DX12, I’m running 60+ fps with Ultra preset and 4K. And that’s with Fenix A320. I never go below 50 fps. The only thing for me is that I do not run any traffic. But that only shaves off about 5-7 fps for me.
At the end, you have to find out what is causing the performance drops. Many times, it’s not the sim, but some add on or something else. Also, find out what works best for your hardware. I found that by disabling virtualization and hyperthreading and even Ecores improved my performance by almost 10-15 fps more. Here is a thread I did on this.