I swapped from a lifetime of android, to a 15PM im not missing much nowadays. The only major feature id say is split screen but PiP does the trick most of the time for that, while not as perfect that and the keyboard being a bit slower would be the only "downgrades" i could come up with in a month of use.
The list of things that have worked better is a lot longer, the bt and cellular modem is miles better than my old pixel 6 pro, the high refresh rate screen actually delivers high framerates more of the time and the apple intergration is so much more cohesive than googles hodge podge of integration
As someone who did a similar swap Pixel 6 Pro to 15 Pro Max, and my first time off android Hope you enjoy it. I do miss the customization but at the same time i did a lot less customization than i did back in the older android days. The stability of the general phone things has been alot better, bluetooth, mobile data and reception has been a big improvement. The thing that really pushed me over is the device values, my P6P isnt worth putting a new battery in a iphone bought at the same time can have a battery replaced for $69 and will be worth about double the pixel equivalent.