iPhone 17 Pro Max screen dims despite max brightness

Then there are 4 additional settings to check, but I don't know what ALL OF THAT refers to.

• Attention Aware Features - Go to Settings > Accessibility > Face ID & Attention > Attention Aware Features, and turn that off.
• Night Shift and True Tone - Go to the Settings > Display & Brightness and make sure both Night Shift and True Tone are shut off.
• If you are using the Always On Display then the dimming of the screen is due to that setting being enabled. You can turn off the Always On Display at Settings > Display & Brightness > Always On Display.
• If your battery indicator is yellow then that means Low Power mode is turned on and will affect screen brightness.

Sounds like you need to go to Settings > Accessibility > Display & Text Size, and scroll down to shut off Auto Brightness. Note that as seen in the message there, it may affect your battery usage.

Try turning of auto brightness in Accessibility>Display and text size>last feature.. That might help