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I can’t say anything about it’s quality, but Wikibooks has German.

Looks like it may be worthwhile as a or an additional starting resource, but maybe not more.

yt-dlp supports various sites and authentication options (may have to take over from webbrowser). I don’t see an explicit extractor in the source, but it may work anyway. It’s what I’d try first.

(Aside from the other issues) A DVD may not even retain it for 15 years

https://en.wikipedia.org/wiki/DVD#Longevity

Hardly something to bet on for continued availability.

It was trained on human text and interactions, so …

maybe that’s a quite bad implication?