I agree completely with every single point in this post and also had the same upgrade path.
The sound is much better and I'm surprised it wasn't mentioned. It's much louder than the 14P was and more dynamic. I think the current 15 Pro is as good as much larger speakers in Apple's older laptops were, it surprised me. If you hold it up near your face in landscape the spatial audio even kind of works now. They are clearly making massive improvements here.
The 14 Pro was probably the worst iPhone I've ever owned, I may have had a lemon but mine got insanely hot constantly, far warmer than the 15PM. It destroyed the battery and I ended a year of ownership at or below 90% remaining capacity. The screen would dim often due to heat (I turn off auto dimming) and I don't use intensive apps. It was also a total brick, two-hand using the 15PM is much more comfortable than 1 hand using the 14P. It's still narrow enough that I can comfortably hold it one handed and reach something in a pinch if I need to, but the balance overall is just a lot better.
The 5x lens is noticeably better than the 3x for me in quality. Maybe this is due to software improvements or the processing pipeline, but I was never satisfied with the 3x on any iPhone (or the ultrawide until the 14) and both are very good on the 15PM. Main sensor is great obviously. Capturing RAW on this phone is also at least 30-50% faster than it was on the 14P, possibly even faster. 35mm as an option for shooting, combining pixels, whatever it's doing, is great. That's my preferred focal length on DSLRs (I am a semi-pro hobbyist photographer) and it's nice having that on a phone in my pocket that I don't have to futz with.
In-hand feel is the best of any large iPhone since the XS Max or maybe even the 7 Plus. It's wonderful. I use no case so the design change matters a lot. I'm very happy with this upgrade, and the larger screen size lets me read research papers far easier - scrolling to get through diagrams was annoying on the 14P.
I think this is close to perfect size/weight for a large phone, lighter would be welcome but the rumors about the 16 Pro getting even larger are not making me happy. If it does get larger I may skip the 16 to see if the bezels reduce on the 17 to make it slightly smaller. The larger width would be too much for me.
I'd still kill for a 16 Pro Mini for one-handed ease of use and carry size for a "phone" and messaging tasks, but for a pocket computer I think this is really excellent and carefully considered decisions were made, almost all of them positive. I'm not sold on the action button yet but I don't hate it, I need more time to figure out a shortcut that would be useful to map.