One of the most difficult parts when you are preparing an exam on your own, is the writing part, because there is not specific guidelines to correct it, or you are not able to correct your writing for yourself.
Surfing the internet, you may find some people that correct writings (not a lot, though) but the majority of them are really expensive, if we consider that for preparing an exam you cannot do only one writing.
Fortunately, Cambridge English has developed a very useful tool called "Cambridge English; Write and Improve"   which corrects your writing for free and gives a feedback in seconds (along with your writing command of English compared with the CEFR).
You can write about the topics that are available in the page (that change every month) and that varies for a wide range of kinds of writings (like a letter, composition, a story…) and that links with the writings that may appear in the Cambridge exams, or write about your own topic (which is absolutely perfect), with an extension between 150 to 600 words.

 https://writeandimprove.com/


PS. I've created a workbook with some writings taken from the CAE exam, the code to join it is YN8ABYFC