At one gigabyte, your best option is redundancy, not reliability. Put copies on a dozen cheap USB thumb drives and store them with friends, relatives, or just in a metal box out in the woods. Upload to Google Drive, OneDrive, and everywhere else that’s offering a free tier that’s large enough. Burn a fresh copy to a DVD-RW every weekend and stash it somewhere.

When you’ve got enough backups, it doesn’t matter if a few of them fail – you can always grab another copy and restore from that.

rsync over a wired network. That way, I can stop and resume at any time.

Assuming your 18TB drive was manufactured sometime in the past thirty years or so, there’s no need. Manually managing the internals of a hard drive pretty much went out of style in the late 1980s.

You say you’ve got PCIe 2.0 slots. If you’ve also got SATA 3.0 ports, the SATA drive is actually going to be faster than an NVMe drive in a cheap one-lane adapter (600 MB/s versus 500 MB/s). The NVMe drive only wins if you get a four-lane adapter and stick it in an x4 or larger PCIe slot, and even then, it’s only going to be about three times faster at best.

No idea about compatibility of adapters. When I was facing the same decision while upgrading an old computer, I went with the SATA drive.

A recertified drive is a used drive with a decent warranty.

There’s no official standard, but “center-positive” is far more common than “center-negative” for power supply connectors.