Really? Here's my suggestions. Remember, as a Thalmor, you have a small set of goals.
#1 - The war must not end until it's ground both sides to a pulp.
#2 - If the world is destroyed, you win, because it will magically make all true believer High Elves into spirit-gods that are immortal.
To accomplish goal #1 - Don't side with either side in the war. Let it grind on.
To accomplish goal #2 - You have two, maybe three chances at this one in the game.
Spoiler
Option #1 - Don't stop Alduin. Let the world-eater eat the world. You want him to win, because that ends the world, and will make you a special spirit-god.
Option #2 - Join the Mage's College...do the quests up until you have the Staff of Magnus. Then just walk away, and let your fellow Thalmor do his thing. Feel free to stop playing the character after that, as the game will never progress to the world being cracked open like an egg, but it'll happen, because you have the only means of stopping it, and absolutely aren't going to do so.
Option #3 - Do the quest to interrupt the binding of Potema...don't do the followup. Let the mad necromancer be reborn, so she can grind Skyrim and the Empire to dust under her undead legions.