After I intensely experimenting all the backup solutions. I would suggest you to upload your photos to cloud like Google Photos instead of iCloud. iCloud Photo Library is not something you want to use if you want to offload your phone because if you delete photos from your phone, the photos will disappear also from iCloud Photo Library. And I believe same goes to the iCloud Backup for your photo gallery (this is what I tested, but someone correct me if I am wrong).
Non-cloud solution is that you export your photos/videos from your iphone to your computer/laptop.