On the one hand I agree that when you are submerged into it you learn faster.
On the other hand I have to state that you need more than just that, you need to actually like it or give a little more than a fuck.
I mean, I know I never put a hell lot of an effort into it, but after 5+ years living in Germany, my German is still topkek quality. Yes, I am "somewhat" fluent(?), yes I "somewhat" understand it, but hell, it sounds like shit. I think it would never, even in a 1000 years, pass as a native speaker's.
Also, when you complain to yourself that (most) anime in Netflix only have German subtitles, it should be a flag that you are not fast enough reading German, compared to English, to the point I even end up either changing the audio to German (eww, but at least it is easier to follow), or using Crunchyroll, ... or pirating the hell out of it when it is restricted.