Could you give me tips to improve my academic writing?

You have a good base, with only minor errors in prepositions and tenses. You're right it is very different from spoken colloquial English and quite difficult - during my master's study 75% of my class spoke English as their second language and all of them had to write very complicated academic papers in English. Some were better than others, but eventually they all learned and it comes with lots and lots of practice. Many told me that it helped them to read many newspapers or general interest articles to get a sense of how professional writers write. My suggestion is to read read read! Make sure it's interesting to you - maybe Forbes if you like business, Wired if you like Technology, Sports Illustrated if you like sports, or National Geographic if you like traveling. Something that may be interesting to you are newspaper articles written in English about your country. For example, my friend from Japan told me that she googled "customs in Japan" or "food in Japan" and read all the articles related to this topic because it interested her to see how other countries understood her own culture. I'm sure that helped a lot because she is an incredible writer in English!