I'm more than halfway done with the German tree, and though I won't be finishing it soon, I was wondering if there were any other sites that teach German that I could use after Duolingo. I've heard many people say that Duolingo can only get you up to a B1 CEFR level, so I was wondering if there were any other sites for learning more advanced German.
ClozeMaster is a site that is like a more retro, grow-up version of Duolingo. In fact the site says that it is basically one answer to the question "what do I do after Duolingo?". It teaches more advanced vocabulary and uses many more sentences, but the main problem with it is that the sentences come from many different sources and aren't always going to be 100% accurate; I've had to file reports a few times because a sentence seemed more like poorly translated Spanglish than pure Spanish to me. Still a good tool nonetheless.
There are also quite a few YouTube channels that offer quirky facts about the German language as well as offer intermediate and advanced vocabulary. Memrise also offers courses of vocabulary you need to get up to a certain CEFR level in a language. I also highly recommend finding sites or apps that offer media (movies, music, books and the like) in German as that will expand your listening and reading ability and give examples of context useful for grammar.
This is my recommendation for you: Watch a YouTube channel called EasyGerman and not just watch it but write the expressions you like the most from these videos, this is as pure as learning a language in the streets of Germany and Austria, full of novel expressions and such.
Get whatsapp and join a German group, here you can post questions and have instant answers about the language, practice pronunciation and make friends.
Another idea is joining polyglotclub.com, here you can write small texts in German and people will correct them for you. The key word to learn any language or anything is just one : Practice, Practice, Practice ... Give a lingot if you like my tips.