Hello,
I think i got it fixed (for me anyways).
I was having the same problem. I just upgraded to a new s9, and Google Photos was saying "Getting Photos" for over 12 hours. So i tried a bunch of different things, like clear cache, uninstall and reinstall with rebooting it every time.
I have over 6000 Pics and Vids on SD, and another 1500 on internal.
What i did: (sorry its long, i didn't want to miss anything)
1)Logged out of Google Photos
2)Went into App Management though the settings, and cleared both Data, and Cache
3)Uninstalled from App Managment
4)Shut Down Phone
5)Pulled SD Card out. (I cleaned it while i had it out, but did not re-install yet)
6)Reinstalled SIM (without SD) and rebooted phone
7)Reinstalled Google Photos from Play Store
6)When i opened Google Photos, it was already logged in, but if yours does not, re-login.
7)Waited for it to say "Backup Complete", which was super quick with just internal photos, like 3 minutes or something.
8)Shut down phone
9)Re-installed SD card
10)Rebooted Phone
11)Opened Google Photos. It still said "Getting Photos", but the status bar was "different".
It originally was saying "getting photos" for about 3 minutes, and then said "backing up 7930 photos. Which i let it sit for about 30 minutes, and its at 2100 photos now. Its not complete done, but close. It for some reason didn't like it when i took the SD out of my S7 and put it in my S9. Thats when google photos started going crazy. But i also allowed the phone to "optimize" google photos for battery life around the same time. So could have been that too.
I hope this helps someone. I was going nuts trying to figure it out. Please let me know if you have questions on my steps.
Good Luck!