Small update. Still working on getting the DB built and data imported, (onto round 9). I'm continuing to see "invalid page in block xxxxxxx of relation base /xxx/xxx" and various data checksum errors, which are presumably from transient write failures. What's weird is that this is present even after I put it on zfs.
I'm forced to conclude that it's a "hardware" problem, so I'm going to try disabling the write cache on the NVMes.
Nothing's ever easy.