There are a multitude of opinions out there, so here is another.
I use two 27" 2K monitors (each 144MHz). 27" monitors are plenty crisp and clear at 2K which gives you performance head room for higher MSFS graphics settings. You can couple that with a single RTX-3060Ti (12GB) which can be found in the $350 range.
I am currently running a 2080Ti with two 27" 2K monitors and I have excellent sim performance at mostly medium settings. And I am talking solid (locked at 30 FPS) performance with stout aircraft (PMDG, Fenix) at third party airports.