Is it ok to use 'Firstly', 'Secondly' and 'Finally' in IELTS Writing Task 2? Absolutely.
It is often used in academic text like this one about insects.
You don't need complex linking words. See the underlined words instead - they give the writing complexity but it is hardly noticeable.
You can easily use these linkers in your writing.
For example:
Members - this reading is Academic Mock Test 3 Passage 2.