Well…first, I would update GPU and, then, CPU - once the sim is not yet optimized for CPU utilization. But as much cores, better.
GPU is a better idea first for some reasons: 1 - having more VRAM is better and; 2 - you have 2 monitors, so you will need a more powerful GPU.
Ofcourse all of this kmprovements will depend on how much do you want to invest at your pc. But keep in mind that: a pc that runs msfs is a pc that runs all the games of this generation, and also an awesome “work pc”.
PS: if possible, how fast are your RAM? 3000 mHz would be a must