Is 91% battery health after 258 cycles normal for an iPhone 15 pro ?

Battery health decline is not linear. It can remain steady for a while, then drop a few points. You are clearly a heavy user of your phone, with so many charge cycles. One thing you can count on, is battery health will continue to decline and when it dips to 80%, it will then be time to pay Apple to replace the battery.

Until that time, don't let the phone go below 20% or EVER fully deplete as that ages a battery faster. Plug your phone in to charge when you go to bed at night and leave it plugged in all night, EVERY night with Optimized Battery Charging turned on. Stop worrying over the natural and normal decline in battery health as it cannot be stopped.

Sounds about right. I suspect you have had it for about 9 months now. The average is about 1% per month drop in battery health. If is not exactly 1% each month and some months may not drop at all and other months may drop more.

Yes, that would be pretty normal as that is a LOT of charge cycles.