You can turn off auto-brightness on your iPhone with a few taps, and manually change the phone's brightness in two ways. Here ...
You can lower the brightness on your iPad even more than its lowest setting in your Control Center by using the "Reduce White Point" feature.
Manual. The brightness jumps around too much when in use as I move around and the angle of the light sensor changes with respect to the room lights. I have mine set at ~1/3 brightness, and it suits me ...