If you don’t play in VR I would definitely recommend a Nvidia 40xx card since the frame generation of DLSS3 is kinda made for MSFS… fantastic technology!
If you do play in VR, a used 3090 is a great choice for MSFS.
I just recently switched from a 3090 to a 4090 and while I do see an improvement, a 3090 gets you most of the way there since you will be limited by the CPU most of the time.