OLED Screen - best battery saving options?

I have an iPhone X on iOS 13.5

On a detailed YouTube video, I found the following suggestions for an OLED display:

Obviously, dark mode and a black wallpaper. Got those.

However, the video was made before the introduction of the dark mode - however, smart invert worked as an analogue. My question is:

Which is better, dark mode/smart invert/a combination of both? I tried the combination - but it seems the system struggles with smart invert - the screen flashes white sometimes which seems to have an adverse effect on battery llife.

Further question: It was suggested Night mode - maximum warmth helps because yellow uses less energy than blue. Seems plausible - but is that even palpable?

Moreover, is greyscale really efficient? Or might it not be the case that an efficient yellow is replaced by a less efficient bright gray?

I would appreciate specific replies; I would less appreciate lecturing (if you need better battery life, buy a power pack/different device etc)

Cheers