Pretty much every processor is mainthread limited on MSFS using DX11. Even the mighty 12900k and 5950x are mainthread limited. In its current form, upgrading to the 5800X3D would likely improve overall performance due to the very high L3 cache, but still likely to be mainthread limited on DX11.
I would urge you to wait a month before making any new purchase. I’ve been testing DX12 on SU10 beta, and it looks very promising.