iPhone 15 battery health at 91% after 254 cycles

Your battery is performing as expected with 254 cycles. You are clearly a pretty heavy user to have 254 cycles in 7 months, but every time you charge your iPhone, your battery health will decline as that is the chemical aging of batteries. On average, you expect about a 1% decline in health for every 25 cycles. 254/25= 10.16. You've only lost 9%, so your battery is as I said performing as would be expected of a battery with that many cycles.

When the day comes that the health of your battery dips to 80%, which it most assuredly will do if you own the phone long enough, it will then be time to pay Apple to replace the battery.

Apple says "UNDER IDEAL CONDITIONS." We don't use our phones under ideal conditions in the real world.

Your phone is performing as expected. If in the first 12 months of ownership, your battery health dips to 80%, you'd have a warranty claim. That is not going to happen with your iPhone. So you are worrying needlessly over the natural decline of your battery, which you can't stop. So stop worrying about it.