I recently purchased an iPhone 12 Pro Max and attempted to transfer the data from my old phone to my new phone. Someone from Apple Accessibility talked me through the process. It went well up until the end when it just sat at 100%, but 2 minutes remaining. I left the phones sitting next to each other with plenty of power for three days, but still got two minutes remaining.
Finally I checked the phones and found that BARD was the delay. The old phone had 53 audio books, but the new phone only had one. Logical as I hadn't told BARD about the unique ID for my new phone, but was still a frustration until I understood the issue. Hoping that posting it here will help some one else.