The fact that you spent 3 days with Apple Support should give you an indication that it is not recommended by Apple. They have clearly stated that iCloud Photos is a syncing service for photos to be available on all devices. Before turning off iCloud, Apple recommends downloading all photos to your device first. If you wanted to incrementally add photos to iCloud and delete them from your device, you would need to download all of your photos again to your device before turning off iCloud, then delete all the ones you don't want again.
The solution Apple provides to limit storage on your device is to use the "Optimize Storage" option. This is the way to go to keep full resolution photos in iCloud, while limiting the storage space on your device. In addition, if you had to restore your device, you would have to repeat your process all over again for every photo you have in iCloud.
The best way to keep your Photos safe now and in the future with OS updates is to use the service as recommended by Apple. It is to be used as a syncing service for Photos to be available on all devices and not as a backup storage service.