Disabling HDR is a must!
I tried this morning - at least even with backlight “80” it gets too dull for my taste again… after 20 minutes, I went back activating it. Daylight instantly looked much more vivid, almost as if I was really standing/driving outside.
When checking my Afterburner monitor from the session this morning, I cannot state any major impact on FPS from my side. But I must add, that I prefer image quality and details over FPS anyway - playing with FOV “normal”, 100% SS, PiTool 1.0, all ingame graphics maxed, SMAA “medium”. I have deactivated only SSAO (looks wrong in VR) and DOF.
Furthermore, as far as I was able to understand, the Truck Sim’s rely a lot on the CPU clocks, especially when it comes to traffic and high density areas such as cities. I don’t know anyone getting constantly 75 FPS in that game even with a high-end OC CPU so far - SCS needs to improve the game’s engine for modern CPU’s and especially with regard to VR.
Anyway, I probably could try to increase backlight to “90” or “100” and try deactivating HDR once again.
You should try with Realistic Graphics Mod as base and finding the brightness, contrast, backlight setting you like
Is installed already from the first day 
As well as Grimes season mods and his “darker nights” 