I was able to eliminate almost all of the brightness flickering while using G-Sync/FreeSync by doing this:
I run a Nvidia RTX 3070 with a MSI Optix FreeSync display. I get around 40-55 FPS in MSFS. My display has a VRR of 48-100 Hz that I changed to 65-100 Hz using CRU. I also set the FPS limiter at 50 and activate V-sync via Nvidia CP. What it does is forcing the display to run at twice or triple the game FPS. Brightness difference between 65 and 100 Hz is WAY less noticable that between 48 and 100 Hz.
Hope it helps!