Very interesting, @JQ - thanks for posting your backup strategy. Hopefully it inspires someone to improve their own strategy. 
I use a slightly different approach, in that I manage more control over when backups are deleted than you do; however, in the end I believe you and I achieve similar results.
My live projects are kept in Dropbox folder, the zipped backups on OneDrive.
My Scriv backup settings are the same as yours, except I “keep all backup files”, as I want full control over when zipped backups get deleted.
I regularly take Ctrl+S manual saves throughout my writing day. At the end of every writing day, I copy the last zipped backup into a separate archive folder for that month. At the end of the month, I clear out the zipped backups from the Scriv backup folder, while permanently retaining the monthly archive folder.
A few times a week I back up my PC to a USB external hard drive.
Every month or two I swap the external hard drive with an off-site backup.
So what I end up with is:
- Live projects on Dropbox
- Zipped backup files from every writing day in OneDrive
- A local external hard drive (with backups up to 2-3 days old)
- An offsite external hard drive (with backups up to 2 months old)
This mitigates the most likely/highest risk issues I would experience in my world (hard drive failure; wildfire burning down the house). It more or less conforms to the 3-2-1 strategy. I can live with this level risk and I don’t find the process arduous.
Best,
Jim