Do I ever need to use a semi-formal tone in General Training Task 1 letters?
The short answer is no.
Chris and the team have done extensive research into how to successfully answer General Training Task 1 letters, and it is clear that the best approach is to choose either a formal or informal tone. You never need to use a semi-formal tone.
In the Writing Academy General Training Task 1 Module 2 - Tone, Chris explains why you don't need to worry about using semi-formal tone and how to very easily determine what tone you should use.
The method for doing this is simple, and it means you can enter the exam with absolute clarity about how to deal with every possible question, and it has helped thousands of students to get the IELTS scores that they need.