If you have the money to spare, upgrade the cpu/mb/ram instead. Otherwise even a 4090 would feel a little underwhelming on MSFS. Maybe in a few months you can score a 3080ti or 3090 for sub $500 for some decent VR.
Read this guys experience with a cpu bottleneck: https://www.avsim.com/forums/topic/625486-poor-performance-with-nvidia-4090/page/11/
I have 2 pcs, one with 5800x3d and 3080 and one with 5600x/3070ti. I only use the later for msfs at around 45fps ultra wide and I’m happy.
The 5800x3d is a beast of a cpu and with the 3080 it does VR well, but Msfs is very poorly optimized for that so I don’t even use it.