How to turn below average sentences into complex/advanced ones?

If you are doing your IELTS/CELPIP/PTE/OET/TOEFL or even school exams or if you just want to write and speak in better English, you need to use three things:
1. Good connectors:
Furthermore, Likewise, Accordingly, Moreover, Other than that , Apart from that, What’s more, On top of that
Linking words:
Firstly, Secondly, Thirdly/Lastly
Comparison words:
While, Whereas, As compared with, On the other hand, In much the same way
Contradictory words:
However, Nevertheless, Nonetheless
Although is used only start of the sentence 
Concluding words:
Hence, Therefore, Consequently, As a result, That’s why, Due to that, Hence why, Based on that
Other useful connectors:
As well as, considering this issue, based on the aforementioned reasons, considering the common worldview, it is apparent that, it can be ascertained that, it’s dramatically apparent that, keeping in mind all of the above, with this is mind, not only, either/or, neither/nor
2. Complex sentences that start with:
Although ______, ______
when ______, ______ 
before______, ______ 
while______, ______ 
until______, ______ 
for______, ______
if______, ______
since______, ______
because______, ______
as______, ______ 
due to______, ______
considering______, ______
based on______, ______
whether______, ______ 
3. Fancy phrases–this really depends on the sentence being made. No straightforward formula here.
Now let’s have a look at some basic sentences:
1. I am writing this letter because I need to make a complaint.
2. Education should be free of cost because it is a considered a basic human right.
3. The weather outside is really chilly and it can give me a frostbite and that will be really bad for me skin.
4. Democracy is better than dictatorship because it guarantees that everyone will have a voice and that is really important.
5. Everybody says that it is important to reduce taxes to help people.
6. The environment these days is getting bad and more bad by the minute.
7. You should really go to the gym and workout since it is important for you. It will help you become good in terms of your health.
8. We need better leaders in the world. They will help us. Governments can do good things for people. We need politicians to do a better job.
9. If you do you best at school, you can get a good mark. Then you can get better career opportunities.
10. Your department needs to work on the sanitary conditions because you are violating health standards right now and it’s bad for your customers.
Here is the. YouTube link that explains how to fix each one of these sentences with a proper tone:
Now try fixing these ones:
11. Sarah will clean up the area because she is really good at it and she can do a better job than me and I don’t have much time.
12. Have you decided on the time when you will perform the audit? I am asking because I need to plan my schedule. I need to tell my roommate about it.
13, Deforestation has become an issue. It is a growing problem. We need to deal with it before it gets too late.
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