To prevent Google backing up photos as you take the photos....I think you will need to turn off backup in Google photos app settings.
I'm not aware that Google photos supports scheduling backup only from favourites. So how do you propose to do that?
To set a photo as favourite is done in Google photos app. Unfortunately I don't think you can see the favourite setting when viewing the photo files in a file manager.
A possible solution for a macro to do what you want could be based on the following
At the start of the day.
Use UI integration actions to replicate the steps performed manually to turn off Google Photos backup setting.
During the day, take photos, set favourites and delete the rest.
At the end of the day
Use UI integration actions to turn on Google Photos backup setting.
Only the favourite photo will be backed up.
Note internet is required to backup the photos.