After discussing it with the family, we decided that being able to load the backups onto a Chromebook was a consideration, since I might not be around to recover the data.
What we wound up with was a USB SATA adapter, and a couple El Cheapo 10TB hard drives formatted with ext4. I’m running rsync of my entire mirrored btrfs onto one right now, with the expectation that someone will be able to read that on a Chromebook, or find a helpful nerd to do it. One drive will live at my father’s apartment. Every six months or so I’ll need to run the rsync again.
At some point one of them will fail, and will have to be replaced with a new hard drive.
We’re also going to start printing out photos and making albums the old fashioned way. I have low confidence that my grandchildren will be able to read an EXT4 SATA hard drive. But I’m pretty sure they’ll be able to see a photograph
This backup strategy is essentially what we were doing before, the big change being that I’m no longer buying these crummy USB hard drives that seem to fail after 2 years. Hopefully the 5.25" hard drives last longer. We’ll see.