The issue certainly could be iCloud related, I thought so too. I've been on the biggest iCloud storage plan for over a year now so no change made to that recently.After some research, I wonder if it is/was some iCloud syncing/caching glitch/bug.
Four years old and no follow-up, but the OP mentions upgrading to a higher iCloud storage tier before the problem started/occurred.Photo Stuck in "Preparing" Mode - Apple Community
discussions.apple.com
And this troubleshooting how-to related to iCloud Photos:
Funny that the same iCloud account has zero issues with my 14 Pro Max though which is completely bug-free and running through all the photo tasks (that choked the 15 Pro Max) like a dream.
Whatever the bug is, it only affects the 15 hardware, not the 14. Both are running the exact same backup of apps and settings.
If anything, this has demonstrated to me how much life the 14 Pro Max has left in it. My battery is at 97% after a year and it never really heats up unlike the new handset.
Going back to the 14 feels like getting a new phone after a week of frustration troubleshooting the 15. Funny how that worked out.
The initial user experience with a brand new product is so important. When the user feels like they have received a ‘lemon’, their appetite and desire for the product evaporates massively. I currently equate the 15 Pro Max with bugs and heat-up issues - that’s a pretty poor perception.
The saving grace is that Apple actually made such a good product with the 14 Pro Max - it shows me that they can actually have great releases. I might actually skip a generation for the first time ever.