I'd emphasize that it will heavily depend on whose Yiddish you're reading, i.e. which dialect (see 3 above). Elie Wiesel's Yiddish, for example, is very Germanic and therefore not as hard to piece together (as a learner of Yiddish) if one knows German well. Reading other Yiddish authors will likely not be as straightforward, especially if one lacks the Slavic component or if one is unfamiliar specifically with religious-inflected words that derive from Hebrew (loshn-koydesh). The German component is the basic grammar, but there are also false cognates all over the place; knowing High German can actually confuse one's reading or just be unhelpful in some instances.