Well, first of all, congratulations on your listening and speaking results. You are clearly very proficient in those skills. In order to get your writing skills up to that level you probably need to improve your grammatical accuracy.
Clearly, your knowledge of English is very good so I don’t think vocabulary is a problem, although to achieve a higher band score in writing you probably need to use a wider range of vocabulary, including more collocations, and a wider range of structures to demonstrate more flexibility with the language.
This will come with more exposure to the written word so try to read as much as you can. Newspaper articles are a great place to start because the language will be up-to-date and accurate. Try to notice examples of collocations when you read and develop a system for recording them in order to remember them and use them in your own writing. Also, look at different structures by analysing the way sentences begin and how this changes the effect - does it make the sentence more interesting or more dramatic for example.
I recommend the Guardian or the Telegraph website because they have a whole range of articles on a wide variety of topics.
