What's the best way to learn a Germanic language?

I just hit 80 million sentences on Duolingo.<br><br>I'm going to take a break from Duolingo after this, because it's not progressing at a decent rate, and I don't think it's very effective.<br><br>The next thing I need to learn is a Germanic language. I want my second language to be German. I will commit to the next course, so I want it to be the best possible course.<br><br>What's the one most effective course for someone with my background?<br><br>I know alpha-phonetic writing, in particular English. I know 80 million words or so in esperanto.<br><br>I have at least two years worth of spare money I can devote to this, so I can buy whatever course anyone recommends.<br><br>So what's the best course for this?<br><br>A British accent is a plus but not a minus.<br><br>&#x200B;