Hi I did A level bio chem maths and french so quite similar to you choices. I would say to get a good grade in french be as curious as you can. You'll find in bio and chem theres a spec with exact things you need to learn but in French you don't have that, so it's up to you to get interested in the topics and build opinions on them so you have something to say in the speaking exam for example. Especially (and I guess this depends on your exam board, I did AQA) but in year 13 you do a lot of politics etc. so its good to follow current events (watching french news is great for this and your listening), that way you're not relying on textbooks that are more than a few years old. (obv this is just one part of it, as said above you need a good understanding of grammar and writing). But overall I really enjoyed french a level and if you enjoyed it at GCSE go for it.
As for Netflix, its a great idea to watch shows in french, I would really recommend 'call my agent' its a completely french show so you get some french culture too!