You covered max screen, white or bright apps, external LEDs on the port, power bank lights, smart bulbs, screen flash apps, projector apps, mirrors. Try a diffuser. Fill a clear bottle with water, or grab a milky jug. Put it on your iPad screen, brighness max. The diffuser spreads light, easier on eyes, helpful for a small room. Set Auto-Lock to Never so it stays on. If you use Magic Keyboard, raise the backlight for desk work.