There are a lot of different non-official English courses on the Memrise platform. Which one should I choose to run in parallel with the official ones? My level is B1
That’s entirely up to you! There is no predefined path or recommendation whatsoever - just look for what you think suits you best.
one thing I use to pick courses is to check what people working on the ones I currently use also learn, which provides me with courses of a similar level and usefulness. And then, as Olaf points out above, I pick the ones that look the most useful to me and my learning targets
You might like to try my picture course: https://decks.memrise.com/course/1346359/picture-dictionary-learn-english-by-images/
Let me know if you have any problems.