Open defaultinput.ini it is located here

C:\SteamLibrary\steamapps\common\Outlast 2\OLGame\Config\DefaultInput.ini

At the ;- Keyboard and mouse section paste the command below to allow you to press F8 to remove the film grain.

.Bindings=(Name="F8",Command="Set OLUberPostProcessEffect GrainOpacity 0")

After pasting this command at the ;- Keyboard and mouse section save DefaultInput.ini. Now you can start the game and use the F8 button and it should disable the film grain.

Every time you boot the game you will have to press F8 for the film grain to be removed. However as long as the game is open film grain will stay off after pressing F8.

Because outlast 2 is so dark and you can barely see in the game you might find yourself using the camera way to often and draining your battery. To help alleviate this you can simply change the battery duration in the config file.

The file is located at
C:\SteamLibrary\steamapps\common\Outlast 2\OLGame\Config\DefaultGame.ini

Open DefaultGame.ini and change the values from default to edited.

Default
EasyBatteryDurationNV=260.0
NrmBatteryDurationNV=200.0
HardBatteryDurationNV=180.0
EasyBatteryDurationMicrophone=260.0
NrmBatteryDurationMicrophone=180.0
HardBatteryDurationMicrophone=150.0
MaxBatteryEnergyForReload=0.3
InitialBatteryEnergy=0.4

Edited x1.75
EasyBatteryDurationNV=455.0
NrmBatteryDurationNV=350.0
HardBatteryDurationNV=315.0
EasyBatteryDurationMicrophone=455.0
NrmBatteryDurationMicrophone=315.0
HardBatteryDurationMicrophone=262.5
MaxBatteryEnergyForReload=0.3
InitialBatteryEnergy=0.4

You could also make the Duration values 9999.0 as this will make the batteries infinite.
After you have finished changing the values save the file and start the game.