If you’re going to upload your photos to Google Photos, why keep them on your phone and occupy storage? If you set so, Google Photos can automatically delete your media files i.e. photos & videos from your Android phone once they are uploaded. Doing so, eliminates redundant copies of the photo. Earlier this feature worked only if you specified Google Photos app to backup “Full original resolution” photos, which used to costs users storage on Google Drive. But now you can backup your photos without the extra cost by selecting “High quality (free unlimited storage)”.
Google Photos’ “Assistant” feature will prompt you to delete images from your phone when the storage space gets low. If you accept the prompt, it will give information on how much space you can free up if you delete photo and videos on the device.
If backup and sync is always turned on, then you can manually delete local copies of your photos and videos too. At the top left, tap the menu icon (three horizontal lines) and select “Settings.” Tap “Free up device storage” to remove original photos and videos from your device that are already backed up.