As stated in the OP Dropbox has become far too bloated, and just feels wrong. iCloud Drive only works 90% of the time, and doesn't play nice with apps that use absolute paths (even many Cocoa MacOS apps, which do this under the hood).
OneDrive is the only client that gets out of my way, just works, and even can automatically move unused files to the cloud without them disappearing from the filesystem entirely. It's crazy to think that Microsoft are making better Mac OS software than Apple.
I haven't considered going the self-hosted route yet, but Syncthing looks promising if you don't need folder sharing or robust iOS support.
Im more worried about my gmail reliance to be honest.
I really only treat these cloud storage services as a sync and as an additional convenience backup. I don't ever think of them as primary storage.
Box is another popular one, which has SSO and other options to allow it's use in more restricted environments. It's at about the same annoyance level as OneDeive.
This guy mentions Drop Box being good in 2012...that's when I stopped using it due to the various bloat and annoyances.
I'd rather have reliable syncing with an Electron wrapper than a fully native UI with broken sync.