please take a look at my idea over here to force auto-brightness to turn off despite no menu option to do so
I found in the settings.fit file there is such an option and I remember from the early HRV days we had the same problem with no menu option to turn on logging so garmin released a tiny FIT file to import and toggle the option directly
I'd like to try setting it to zero to see if it disables adaptive brightness on AMOLED on FenixE/8 series