Is the phone actually running out of free space, or is it just a suggestion from the Google Photos app?
App list > Files - Tap the hamburger menu top/left to see free space on the phone and the SD card (if any).
Google Photos is the gallery app, and the option in to free up space in Google Photos should only be available if it has already backed up the photos and videos (and it should only remove local photos and videos that is backed up).
The advantage of the Google Photos clean-up is that only thumbnails are kept on the phone to save space, and the actual pictures and videos is downloaded from the cloud when needed.
The downside is that more data is required...
Photos and videos can be viewed (and edited) at photos.google.com on a computer after login with the same Google account as used on the phone.
I copy photos, videos and other documents to a computer via USB to maintain a local backup of important stuff, in addition to Google's backup features.
An extra backup doesn't hurt and may be useful some day.
15GB is plenty free space for thousands of photos on the phone, so there is no need to hurry.
Please take the time to carefully read how Google Photos and its cloud storage works: - Select Help in Google Photo's 'hamburger' menu.
The menu is also where you find photos that is moved to the trash can in the camera app. It is kept in the bin for 60 days before it is permanently deleted.
(Google Photos also has an Archive option, for photos you want to keep but don't want to always see on the screen).
I suggest ask a real life friend for help if you are still in doubt after reading the help in Google Photos. It takes some time to get used to.