I don't know anything about how the OneDrive iOS app determines what to automatically upload in the background when you enable this in the app (I don't use OneDrive). It sounds like OneDrive doesn't just scoop up your photos when this is enabled but sends them up as you take them. I would imagine there is some ability to tell it to send the rest, but I have never used the app.
I would not recommend turning off iCloud iOS backups unless you are using iTunes to backup the device. You will lose any app settings, saves, etc.... in the event of a device failure. The backups themselves only backup custom data (ergo apps are not included in the backup because they get re-downloaded from Apple during restore)
Thanks for all the detail here too. I did not say it clearly being too lazy to go look up the precise name of the setting but I turned on iCloud Library on both the iPhone and iPad when I got them which was about a week apart almost 1.5 years ago now I think. So library sizes should be nearly identical but they are way off. My interpretation was something is wrong with that but I don't know what and I am sick of dealing with broken stuff. It should have always kept both devices and iCloud itself in sync but that did not happen. I have no idea why. Either I tripped some strange bug that requires a unique set of circumstances to make it happen or I screwed it up sometime along the way somehow on my own. I don't know how I could have done that though since I set it and forget it for that. I rarely mess with settings in iOS at all. In fact, for months I have been meaning to go prune the stupid number of notifications I get but I haven't gotten to it.
Yeah, the device backups stay on. I can drop iCloud back to 5 GB free and it'll support that. I don't need anything else on there.