The Atronach is a great birthsign, but it really depends on your character. If your crusader is going to be casting spells heavily, I'd advise you against the Atronach. It's the best birthsign for purely melee-based characters who don't rely on magic to win fights. That way you aren't gimped by not having magicka regen and you have a good start to 100% immunity to magic early on in the game. You
can play an Atronach mage, but it's a lot more difficult than a mage of a different birthsign due to the magicka regen gimp. You'd need to really concentrate on getting a large enough mana pool to be efficient in spellcasting. For a mage, I wouldn't pick the Atronach.
As for Alchemy as a major skill, no no no no no. I'd never put that, even if I did use the skill heavily - especially if I used the skill heavily, actually. All my characters are masters in Alchemy, but none of them have it as a major skill due to the fact that it levels like Chuck Norris on steroids.

Not a pretty picture if you're trying to level at all efficiently. Alchemy is one of the most useful skills in the game, hands down, but don't make it a major skill unless you don't want to use it or you want to run the risk of ending up with a severely underpowered character. Alchemy levels so fast that it'll be your first Master skill in no time, if you use it. Making it a major isn't worth it, in my opinion.
Illusion also levels pretty fast, but I've personally had no problems with Illusion as a major skill. I've heard of some people who've had some though, so I would rethink that a bit if I were you, even though I can personally testify that Illusion is fine as a major skill.
Other than that, looks like a pretty good build to me.