Hi. i do have exactly the same problem. I even reseted whole iphone and configured it from zero and still photo library cannot sync due to being stuck at deleting shared library. This happened when i shared that new library with iphone on ios 15 and then deleted it as it was not working. Now my photo sync is stuck on iphone and mac for few weeks. Here also people have this problem but no solution. https://forums.macrumors.com/threads/shared-photo-library.2352670/page-3

After todays beta 7 nothing happened but suddenly later today i spoted that my lib is finally synced and my photo lobrary is unstuck ! Finally after 2 months wow ! You guys have it fixed as well ?

Well i have fixed this problem. After all devices updated to stable ios i deleted the sharing library on both devices. And then created it again and it worked and still ok.