Actually I used config file after I found this, but the calculation of zoom sensitivity still was wrong.
I'm using ironsight pistol and FAL rifle in the beginning of this game, it feels different from other games.
I think the calculation of zoom sensitivity is about 30% faster while using overall fov=103, and about 50% slower while not.
I use 0.9 hipfire sensitivity on csgo, and this game converts 0.9 hipfire sensitivity, too.
So I tried to use 0.9 zoom sensitivity in far cry 6 config file, it worked surprisingly well(only used ironsight).
There must be something wrong, hope you check it again.