Reverse tree completed (yay!), now what?
Grammar, is by far the most difficult thing to master here, any tips? And where can I find reading materials for intermediate/advanced level? Any tips for improving my vocabulary in both German and English I guess?
Congratulations! Short answer reading, videos, TV, person to person if possible. I don't do German but on Amazon (and likely wherever you buy books) I find doing the search in your target language gets better results. For example I search "histoires français" or" la science fiction français".
I'm sure others will have better specifically German suggestions for you.
Beelinguapp has German. A collection of short short stories with audio and translation that can be added or removed with a touch.
There are lots of german fairytales online as most are now out of copyright. Try Brüder Grimm to start (there are complete works available for free on Kindle or project Guttenberg); they're often great but the language and spelling tends to be a bit archaic. Otherwise try books you're familiar with such as Harry Potter.
Congratulations! If you are looking for reading material, you can check out bookstores (at least near where I am, they have sections with different languages, including German) or you can google digital reading material. German TV shows and sitcoms can be quite helpful as well. (There's a pretty good German sitcom called Extra that my teacher showed us) or you could move on to a different app and see if you can pick up on anything new there