I'd recommend MultCloud or CloudFuze. Both offer syncing between different cloud providers and have free as well as paid plans. If you prefer FOSS, try Rclone, though it's a command-line tool and may require more technical knowledge.
I always used rclone on a VPS/seedbox myself, if I ever needed to do anything like that. It's always worked perfectly. Or course you can run rclone locally too, but if you're aiming to do cloud to cloud syncing then some sort of VPS would definitely be better.