Usually if you have to ask, it means you've already been considering it. It is your mind's way of telling you to swap the game out and ultimately you should be pay attention to your hunches. If you regret selling TZ, you can always buy another. In your instance there are many factors pointing you in that direction.

Since you play 10 games a year, that equals a few hours of one day. So if you find one nice TZ to play within a year, you can get your fix. That is achievable and should be a valid solution. Now if you had to play TZ once a month, it might not work out so conveniently. Also since the game in question is TZ, which they made of 20k of, it is easier to find one to play than if your game was Magic Girl, where you may never find one to satisfy the itch.

A temporary trade is a great option if you can get what you want - MM or MMR. Otherwise it is merely delaying and not helping the inevitable selling of TZ. You've played and enjoyed MM in the past. The game hasn't changed and I bet neither have you. You will still appreciate and enjoy owning MMR. So the stars align to support you pursuing the new dream. You know what you want and need to do, good luck.