If your Yiddish is now mostly Hebrew, then you’re not speaking Yiddish anymore.
BS”D
Then all the more reason to preserve it because it is a special haimish Torah Yiddish that is one more boundary between us and assimilation. However, I agree that it should not be forced upon anyone as a first language but rather as something special for use within Yiddish speaking communities or to be able to speak to the older generation.
a course in Yiddish
Do we have such things? Most Yiddish seems to be learned at home or by teitching the Chumash.
In the end it is Lipa Schmeltzer who is preserving Yiddish for us by demystifying it and making it something young people want to learn in an informal way.