iPhone 15 Pro Max Evaluation

I recently made a jump from android to Iphone, buying a second hand iphone 15 Pro Max

Battery health: 88%

Cycles: 690

Color: White Titanium

i have gotten it checked and also have keenly checked and the whole body doesnt have a single scratch, fade, mark, colour inconsistency, and actually looks brand new, even the back glass and so does the screen

i am getting about 8.5 hours of screen on time with moderate use (youtube, insta, whatsapp, chatgpt, spotify)

have kept 120Hz, always on display (with wallpaper) on and my brightness ranges from 25-30% mostly occasionally going to 40% or 50%.

have updated to ios 26 and used the camera somewhat to record 4k 30-60 fps video and capture photo

price was about 600$

I have a few questions:

A: Do you think i got ripped off??

B: As a new IOS user what do you think i should do?? (I am a pretty hardcore mobile user like i have rooted, ported OS so try to wow me)

C: Is there a way to realistically maintain my 88% Battery health and also can i like also increase my SOT??? (NO I WONT DISABLE 120hz or AOD)

btw i have ran diagnostics of apple and it passes all with flying colours and the only indication of it being used is the battery health.....