Hate to be the one to break it to you, but the reason there aren't any automated scripts or known ways to retrieve _raws at the moment is because as far as we can tell, tumblr's actively blocking access to _raws, at least from their previously known location.
Whether it's because of something entirely unrelated server code change or because some patreon retard didn't realize that you could pull full quality images from tumblr and complained to take it down, we don't know. Tumblr themselves aren't saying anything (actually informative) about it either, so who knows what's going to happen.
Personally I'm holding out from updating the self generated tumblr artist archives again until a workaround is discovered.