Koh Tao offers year round diving, and therefore the dates or season of your DMT shouldn't affect your employment prospects too much. What will affect your employment prospects will be where you do your course, and your personality and attitude throughout the course. Despite what others say, freelance DM work is abundant around Koh Tao for the right divemasters, but newly qualified DMs tend to find work or jobs easiest at their dive centre of training. My advice would be to find the right dive centre with the right instructors and qualities (facilities, programmes, equipment, eco, atmosphere). I did my DM and IDC with Master Divers in Mae Haad, they prioritise quality training over quantity and provide the best courses on the island. Check them out if you're serious about working in the dive industry.