Here’s what I do:
Dropbox - contains active Scrivener project folders.
OneDrive - contains inactive Scrivener project folders, zipped backups, and backups of various customized Scrivener stuff (Options, Themes, compile formats, templates, etc.) OneDrive is relatively popular with posters here as storage for backups, but pretty much any syncing service will do for these types of files.
External backup drive - 3-4 times a week, back up everything
In case you’re interested, here’s a post I wrote that has specific advice for zipped backups - see the BACKUPS section.
Best,
Jim