I had almost 240gb to back up from the old phone. Could also be an issue with my Mac, as mine is late 2015 and I've had issues with plugging into the USB port before where it'll act like it sees the phone, and then disappears, then reconnects, and just keeps doing that. I had to plug into the USB port on the back of my Apple monitor to get it to stop.
I also recently had issues with Photos and not being able to sync from the iPhone to there or to Image Capture. Finally got fed up and purchased iMazing and was able to get my photos off with that app no problem. Thinking I might have to back my old phone up with iMazing and restore from there as opposed to the stock solution.
Really frustrating that this phone arrived yesterday afternoon and I am still fighting with it.