You might want to have a look at rclone. Not sure if it does real-time sync, but it’s very capable. No GUI though, at least not in the official distribution
I'm working on a similar app https://cloud.doshare.me we will open source after beta testing.
Because cloud storage clients do suck currently, including Drive, OneDrive.
Integration with OneDrive is a feature after beta phase we want to implement.
