Absolutely doable! Your active .scrivs must be on Dropbox if you want the easiest way to sync automatically with iOS/iPadOS devices. But your backups from Mac and PC are best zipped with the date included in the name but can be stored on any of your other three cloud services. If you do store them all on one cloud service, I’d raise the number kept to 25—which is not a bad idea in any case.
Especially, if your projects are big, running into hundreds of megabytes, I would store the Mac backups to iCloud and the PC backups to OneDrive to maximise available space and security.
Mark
PS My set-up
I don’t use the iOS/iPadOS version any more.