Transfer IPhone 12 Pro to iPhone 15 Pro Max

Trying to transfer all of my iPhone 12 Pro to my new iPhone 15 Pro Max via the iCloud backup and Bluetooth. Out of the box the 15 said it had a software update (to OS 18). I selected to go ahead - which may have been the problem. It has been hours waiting for the two phones to “transfer” data. The upgrade took a long time and then it scheduled the install for midnight. In the meantime I now have some use of the new phone but I believe it is still transferring data. My mail accounts have not appeared on the new phone - and items like my music library have only partially appeared. I have no idea if I should cancel - and if so, how do I get back to the start of setting it up to mirror my iPhone 12?