Some ideas how to change Google Photos settings:
iOS/iPadOS Settings > Apps > Google Photos > Photos > None.
After doing that Apple Photos is disconnected from Google Photos but (unlike the page below says) you can still access Google Photos Collections and Memories by clicking their icons in Google Photos app or via a browser.
Also Disable Back Up & Sync in Google Photos app > Profile picture > Google Photos settings > Backup > Backup OFF so Google Photos does not nag about it.
https://www.wikihow.com/Delete-Google-Photos-Without-Deleting-from-iPhone#
https://www.wikihow.com/Download-an-Image-from-Google-Photos-to-Your-Computer