Haven't done any of your subjects (Y12 studying Maths, Further Maths, Economics and Physics), however I started studying mine earlier as well last year (although quite different circumstances), and I am now predicted 4 A*s so I would recommend it. What I did:
-Buy textbooks, especially for History (I have heard the content is a significant step up from GCSE).
-I wouldn't advise making your own resources apart from notes as I find it time consuming and a waste of actual revision time, but I don't take subjects like yours so in your case it might be worth making flashcards - in fact I would advise making them from the books.
-You should 100% look through the specification, this is the first thing that I did, it's quite stunning how many people don't think to do this. Looking for essay subjects will definitely help you.
-I don't need to be a language student to tell you to practice your German, nor will I patronize you by explaining why this is a good idea. Maybe watch some German movies during lockdown, or read newspaper articles in German online? If you know anyone who is German who you are close to, talk to them in German.
-For literature, read your set texts - I read each of mine 3 or 4 times and that was only for GCSE.
Good luck