The setup is simple, it runs Debian with four drives. The OS and primary data is RAID1 then two independent drives that a script automatically keeps everything in sync so I have three copies of everything over four drives.
I also have two USB flash drives with it (but not connected) that have an identical OS setup on so should the local system drives die I can ask my mother to pop in the USB and boot it up and still access the 2nd and 3rd backup drives. It all works when I have tested it out although never needed to actually use it.
This isn't for multimedia of course just my really important stuff such as 2FA recovery keys, scans of passports and legal paperwork, etc. As it is only a few GB I don't bother to delete anything ever, I only ever add so even if I rescan in stuff it doesn't replace the old copy.
It is all encrypted on my end first so even if someone steals the machine the data is as safe as it is anywhere else.
Is it perfect? Nah but it is more than good enough and hasn't failed me. I also have local backups of course, this is really my last resort kind of thing.
The credentials for accessing those backups get encrypted and backed up at a relative's house.
So you almost never need to update the data at the relative's house. Although you might want to check they haven't lost it from time to time :)