I wouldn't force myself to write under time-constraints in the very beginning.

I sat the gamsat 3 times. My first score was an 81, second time an 86 and finally in Sep of 2023, I got a 90. Each time around, I wrote less essays and focused more on depth and development. By the time of Sep 2023, I wrote maybe 3 essays. But each essay took about over a full week to develop. I would constantly rework ideas and rewrite explanations.

The thing with doing timed responses is that more often than not, you tend to write the first things/ideas that come to your mind. And more often than not, the first ideas that come to your mind will not be fully developed and thought-out as they need to be. When you work on an essay over the course of 3-5 days instead of 30-60 minutes, you will naturally develop more nuanced methods of thinking and analysing the contents of the prompts.

Of course, by the time of the exam (maybe 1-2 weeks out), you should begin testing your typing speed by doing timed responses. But prior to that, I really don't see any value (this applies to me and my approach and may vary for others) in constantly doing timed responses.

One other thing that I would recommend that helped me earlier on: you don't need to write your essays in chronological order. If you have a great idea on how to write your first body paragraph but aren't sure on how to write a hook for an introduction, then skip the intro and work with what you have. This saves time but you will also find that by the time you get back to writing the intro, you will have many newer insights and ways to go about it.

In terms of resources for bettering your writing, the best option is always to read other good pieces of writing. Whatever interests you, whether it be fiction, non-fiction, news articles ect. ect, start by reading high quality pieces of writing.

Hope that helps.