When it comes to variants, I want to try and get the most variety for the fewest number of variants. Hence, you wouldn't see a Yak-9D if I did a 9M because it would be completely superfluous.
The Yak-9M is a great suggestion because it requires very little resources to make. A few minor cosmetic changes to the shape and a new skin. Much of the flight modeling work would already be done because of the -9T.