how do you make sure that EVERY FILE in each copy is good.

Checksums.

Personally, I use TeraCopy to safely copy a folder/file from my main drive to my backups (there’s even an option on there that will save a checksum of said folder/file on the backup so that I can later run that checksum and see if anything has corrupted).

What do I do if there’s corruption? Simply delete the corrupted files and replace them with good copies from other backups.

Like plunki said, YT-DLP will work. All you have to do is feed YT-DLP your playlist URL (if it’s private, I believe you’ll need cookies but if it’s set to unlisted, you won’t need them) and it’ll begin to download your videos.

I’d also highly recommend adding the argument “—download-archive archive.txt”; this will create a text file and log the videos you download so that when you run the command again later, it’ll skip all the videos that it’s previously downloaded and just download the new additions.

Only issue is that it isn’t automatic by default* so you’ll have to run the command every few days/week.

*While it may not be automatic by default, you could whip up a script that runs the command every few days or week w/o your input.