You all probably already know but just in case: there is an opensource allowing to use FSR on VR. It does mod the OpenVR.dll used by Steam VR. (implementation in DCS desktop mode still needs to be handled by ED for it to work with the Game graphic engine.
Here the link to the GIThub https://github.com/fholger/openvr_fsr
I've tried it with Pimax 5K+ (in the normal Field of view mode, 72Hz) and that's improving performances to a point where I can fly an F14 in multiplayer at Ramat David Airbase with Pixel density at 2 (smooth, stable 45/50FPS) and almost smooth at 2.5.... (40/45FPS).
Even though I do have a solid config (5900X + 6900XT + 32GB) there is NO WAY I could have managed that without FSR without drastically reducing the graphics quality (here I'm only turning SSLR SSAO off MSAA x2, no SSAA).
If you are in VR you have to try.