Hello @RebootRenegade ,
Most apps that are not from Google looks like for a photo that is on the device. As Google photos and stored with in their app data, Only way to use those picture with out downloading would be to implement google api connection which can be a challenge considering android tries as much as possible to not share data between apps at that level.
Currently only option to sync photos from gallery is Samsung cloud. If you are ok to use this option, you phots do remain in gallery but can be backed up to cloud too and allows sharing in 3rd party apps too.
One UI Beta Team