Unable to dim screen at all

Hey there Captain Scout,

Thanks for reaching out to the Apple Support Communities.

Just to confirm, if you manually try adjusting the brightness from the System Settings, does the same issue persist? If unsure, give that a try with the steps below: Change your Mac display’s brightness - Apple Support

Manually adjust brightness

1. On your Mac, choose Apple menu  > System Settings, then click Displays  in the sidebar. (You may need to scroll down.)

2. Drag the Brightness slider on the right to adjust the brightness of your display.

Depending on the type of display connected to your Mac, you may also see a Contrast slider that you can use to adjust the display’s contrast.

If unable to manually adjust the brightness, then based on the steps you've already gone through, we would recommend reaching out to Apple Support directly from this link here: Contact - Official Apple Support

Take care!

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Also have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?