@Richard Berry, Thanks for posting in Q&A. Based on my researching, I didn't find any built in policy to change the screen brightness. But I find it seems you can use PowerShell script to do this. You can test the script in the following link on one device to see if it works:
Note: Non-Microsoft link, just for your reference.
If it works, you can try to deploy the script via Intune.
https://learn.microsoft.com/en-us/mem/intune/apps/intune-management-extension
Hope the above information can help.
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