Check out Joerg Sprave’s video [1] on this with him and the IG Metall lawyers. Joerg is the slingshot channel YouTuber who is helping lead the charge on this. In the video, they give a good breakdown on their demands and their strategy.
The false self employment concept seems fishy to me. I doubt they will succeed on that front.
Perhaps they will be able to get somewhere with GDPR, but, supposing that they are able to get the courts to classify the judgments about uploaded videos as personal data (a stretch), YouTube will just begin disclosing that information (or stop retaining it). If all YouTube retains is a bit that says how many raters rated a piece of content as "teen" or above, what recourse does the GDPR offer? Sure, you can have that information, but that doesn't mean that YouTube is required to disclose the mechanisms by which they made the determinations.
Their last option (collective action, like strikes or what have you) is just wishful thinking. Most YouTube watchers do not care that a guy who is getting paid shittons of money to make videos about slingshots is making slightly less shittons than he otherwise would.
[1] https://m.youtube.com/watch?v=oZZ5Kouj_hQ