(Can we make RAICSP a thing? Or do multi-cloud solution vendors already has a snappy marketing term for storing everything on all of S3, Azure Blob, and Google Storage at once?)
I strongly suspect (as in “for the right price would stake my life on it”) that there are some common failure modes across the major cloud vendors’ offerings.
Your advice is directionally sound (and is what I do for family docs and photos); I just think it has fewer meaningless 9s than you’re imagining.
I believe Resilio can do it, but it is closed source.
If you're looking for backups and not dropbox-like functionality, then something like restic might be more appropriate. It does actual content addressed backups, so you've got history for those times when you realized you messed something up six backups ago.
Just went through this with my family (in Australia) when the local bank branch said they were shutting down the service. It was basically a locked room in their branch, there was no "safe" involved.
If my backup harddrive is in the same house as my main storage and my house burns down, I'm fucked.
If I'm fancy, I may have multiple backend storage providers, too. Lot easier to do that then to have multiple houses. I wanted to use a safe deposit box in the past, but last time I called banks near me, they didn't even have safe deposit boxes available to rent! I can ask some friends, but then will be limited in frequency of updates, still.