I'm not even sure what my rule is.
I use iCloud, OneDrive, and Sync on all devices. Photos are in iCloud.
All cloud services are set to fully download to an M1 Mac Mini that functions as file server/Plex server.
That Mac Mini has a Sabrent 5 bay enclosure attached with an SSD and two hard drives in it.
Everything on the SSD gets copied to the first of the hard drives nightly, and then that hard drive is cloned to the second drive.
Used to have a fourth drive for Time Machine but it died, will replace it with a Black Friday deal, maybe.
The entire Mac Mini is backed up by Backblaze, even the Plex Media.
I have random old hard drives lying around with the most important stuff (photos, docs) back up, and I am in the middle of preparing M-Disc backups of this as well.
Future plans:
Still might get a NAS if I have money burning a hole in my pocket.
Going to get a third 8TB hard drive on Black Friday and do a monthly backup, stored at the in laws, rotating with one of the drives here. I used to do this with a safe deposit box but it cost too much and was too much of a pain in the ass to get to.
I’ve gone so far that I’m scanning my books. Almost done. DVD’s have been gone for years.