The main reason I wanna use iCloud for my photos is just so that the pictures on my phone are backed up,
iCloud Photo Library is not a backup, it is a syncing service. It is syncing the photos between your devices and making the photo libraries identical on all devices. All photos from all devices will upload to iCloud Photo library and be merged into the same library, and then sync back to all devices.
When you enable iCloud Photo Library on your iPhone, the iCloud backup of your iPhone will no longer include the photos. You will not have a backup of the iPhone photos at all, unless you make a backup copy of all photos on your Mac with Time Machine.
- If your main concern is to have a backup of your photos, disable iCloud Photo library on the iPhone, so all photos are in the camera roll. Then back up your ipHone to iCloud regularly. These backups will include your photos.
- If you want to use uCloud to sync your new photos to your Mac, use My Photo Stream or download the new photos using a USB connection.
Is there any way I can select which pictures are being synced?
One way to use iCloud Photo Library to sync only selected photos is to have more than one Photos Library - a large archive library with all your photos, that is not syncing with iCloud and a smaller, new Photos Library with your favorites and the most recent photos. Only one library, the one you designate as your system Photos Library can sync with iCloud Photo Library.
See: https://support.apple.com/en-gb/guide/photos/create-additional-libraries-pht6d60 b524/mac
and: System Photo Library overview for Photos on Mac - Apple Support
