In addition to @DTLow’s and @Blanc’s comments, my view on putting DEVONthink’s Zip backups into a third party syncing cloud service, e.g. iCloud, Google Drive, Dropbox, etc. … indeed the copy and sync works (or should work). And should there be need to retrieve a one or more of those files, it might be the first place I’d look as it would be “handy” if lost on desktop.
BUT, as these are third party sync services, then if in the past, perhaps without even noticing, on the desktop device(s) these files were lost/corrupted/deleted/whatever, then on sync the copies of those files on the servers would be lost/corrupted/deleted/whatever on the next sync after the “incident” proving them to be worthless in the intended purpose. They aren’t backups. I accept that some think they are.
Backup Strategies - #2 by Blanc a good strategy. I do something similar. Using the “3-2-1 Backup Strategy” is always good. 3-2-1 backup strategy - Google Search