I really wish this were available. Smugmug seems to be fairly easy to work with. Quite a few 3rd party tools are listed on their site. I don’t see a true syncing tool available, the api may have limitations that prevent it. Still, it would be great to have a more capable method of uploading photo & video backups.
Here are some of the 3rd party apps that work with the service.
https://help.smugmug.com/third-party-uploaders-downloaders-migration-tools-and-utilities-r11dlxDJNBz
This is a syncing tool that looks promising but its command line.