Then I discovered that when Carousel is installed, the Dropbox app stops automatically uploading photos from your phone. You have to now open Carousel to do so. Broke my existing upload process, and due to iOS permissions for the new app or not launching it frequently or something the new Carousel upload process wasn't reliable.
Discovered this by noticing that weeks of pictures weren't backed up to Dropbox. Glad I didn't lose my phone.
Ended up just deleting the Carousel app rather than figuring it out. The Dropbox app started syncing reliably again.
Carousel's "Flashback -- Discover This Day in the Past Year" feature strikes me as a very clever social workaround since it gets you to open the app at least once a week.
The Dropbox iOS app has the same limitation. Presumably you opened it more to do unrelated tasks, so it makes a lot of sense to bundle them.
Google Photos has the same limitation as well on iOS. I find Carousel's syncing and Google Photos' syncing seem to be roughly on par with each other.