Synology DS1517+ $700
WD NAS 8TB Drives 5x $250 ($1250)
$1950 one-time
~$60/mo for Amazon Glacier cloud backup of entire NAS
This is not my final answer, but this seems to be the best price/value for what I want. (I want to not have to worry about this again for at least 5+ years).
The only question then would be single-disk redundancy vs 2-disk.
I’d probably go with single disk (RAID-5 or equivalent), but also buy a 6th drive to keep as a cold spare. I’d rely in this case on a weekly or monthly Glacier backup to cover the unlikely scenario of two disks failing inside of the time it takes to rebuild the array from a single drive failure.