Photos Uploading to iCloud

What you're doing is failing to understand how iCloud Photos works - it doesn't back up your photos, it merely syncs them across devices. Apple really needs to make this far clearer, as many believe as you do, and this forum is awash with posts from people who've deleted photos from one device, not realised that they go into the 'recently deleted' album and are then permanently deleted after 30 days. If you delete photos from your phone, iCloud Photos updates to reflect the changes and the changes are mirrored on any other Apple devices you own.

Learn what iCloud backs up by reading this Apple support article: What does iCloud back up? – Apple Support (UK)

If you wish to clear space on your phone by deleting photos - and they're photos you wish to keep - you will first need to create a backup. You can then save the backup to iCloud Drive (but you'd be well advised to have a physical copy on an external drive).

Otherwise, you'd need to delete apps you no longer need if you need to free up space.

I believe that your phone won't sync photos while you have Low Power Mode enabled. Try turning that off, make sure your phone is connected to WiFi and plugged in overnight. See what happens.