Please do not use a sync service that is not designed to back up your data and retain its integrity. If you unintentionally delete a file and it doesn't stay on the service forever, it's not a backup service.
iCloud and Dropbox are not backup services. BackBlaze, CrashPlan, and others are dedicated backups service. However, be careful with external drives backup, some of these services have inactivity timer for external drives. I'd also recommend a backup service where you can provide your own encryption key instead of relying on the service to hold it. Otherwise, any data are exposed to them.
The business version of Dropbox might work but it would be expensive compared to the BackBlaze or other services.
My suggestion is to use an external drive or a NAS in your home network to back up your data via Time Machine. Not only is this local to your network, it will be faster to restore as the cloud services are not likely to max out your bandwidth for restoring.