I use iCloud as my primary storage, 200GB tier shared with my family. Running low so a 2GB upgrade is due. Photos also backed up using Time Machine on my MacBook.
Up until recently I relied on OneDrive as my secondary offsite, cloud photo backup until I found out the OneDrive iOS app was transcoding my HEIC photos to .jpg for backing up. Despite HEIC being reasonably well established and supported by iOS since v11, OneDrive still has an issue with it. Beyond Office, Microsoft really do seem determined to fumble the ball at every turn. No sign or suggestion that the iOS OneDrive app might support HEIC any time soon.
I’d kept Google Photos around using the free ‘high quality’ tier, and recently switched to Original for backing up from my iPhone. As well as supporting HEIC natively, Google Photos is a pretty impressive app/service, useful for sharing photos with Android only relatives. I’m tempted to pay for the 100GB storage tier as I’m down to <2GB free on my account. Could just do with a way to clear the older ‘higher quality’ photos and have them back up anew at ‘original’ quality.
Seconded turning of background app usage and only backing up when both the app is open & in use, and then only via WiFi.