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.
Most lossy formats these days use variable bitrate encoding: fewer bits for low-detail parts, more bits for higher-detail ones. Forcing fixed-bitrate on one means lower quality in the action scenes because you're wasting bits on fades and talking heads.
Tar.
Mostly not joking here -- the image and video formats you list are already heavily compressed. You'll be lucky to get even 1% compression from any format, so you might as well just package them up in an uncompressed archive format.
I'm sure you can find some Chinese company selling them, but there's no point: the Gutmann method is designed for hardware that hasn't been made in the past 30 years. With modern hardware, a simple zero-wipe will stop anyone short of a three-letter agency, and even waving a degaussing wand over it will stop most attackers.
If the number of bad sectors is a multiple of 8, the software is probably counting logical sectors of 512 bytes each, while the disk has physical sectors of 4096 bytes each. Each time a physical sector fails, the software counts it eight times.
Still worth replacing the disk -- a functional disk shouldn't have any bad sectors.
QNAP NAS at work syncs to externally mounted disk once a day (one way mirror)
You can get cheap versioning here: instead of syncing to one external disk, get another disk or three and swap between them on a regular basis. As long as you realize that ransomware has encrypted your data before you've cycled through the entire set, you'll still have an old backup you can recover from.