605/605 Split is my favorite, and most played character, and I recommend it no matter what vocation you're trying to play as end game.
1-10 Mage 10-100 Assassin 100-200 Sorcerer is indeed the best way to go. Base defense is useless at 200, and 3500 Health is enough to survive anything. Your plan regarding leveling should work as well.
Now, my most played class is MK, and I dress for looks, my physical defense comes out to about 610. Nothing that could consistently hit you is going to kill you at an unavoidable rate. I have NEVER died with 610 Defense at Lv.200
Lv. 3 Armor is going to give you 800-1000 (Yellow/Red Vocations) Defense, and you're as good as invincible at that point. For Blue classes, 700 Defense.
I made a balanced build that eventually became my main. 600/550 in physical/magick attack and 320/320 for defenses. It was tedious, especially because most balanced builds will have you getting a bunch of post-100 Sorcerer levels, but I liked watching the build come together and I've been enjoying it for months.