SOLVED: Photos stuck at "Preparing to import"
TLDR: "Reset all Settings" on your iPhone to fix Photos stuck at "Preparing to import"
After searching around and finding no answers that worked for me, I contacted Apple Support and after over an hour of troubleshooting across multiple reps arrived at the solution that resolved this in my case: on the iPhone, "Reset All Settings" (Settings | General | Reset | Reset All Settings), then reboot the Mac, launch Photos, reconnect the iPhone, and try again (after giving a few minutes for the settings to reset). I'm using an M1 Macbook Air running Mac OS 11.1 Big Sur and iPhone XR running iOS 14.4.
Thought I'd share the results of my efforts with the Apple Community. Hope this helps, keep well, and stay safe!
MacBook Air