Don't go illusion just for invisibility, you can do that with alchemy. And upping illusion for spells will prolly make you put some points in magicka, which is a waste. Though, not much options what to raise anyway with a sneak.
Sure, you have to put points in magicka, but does it really matter THAT much for an assassin character?
Assassins aren't going to get hit that much so health is much less useful than it is for other classes. It will generally come down to this: if you get detected by a strong enemy, you die (even if you put in a bunch more health), if you don't get detected you win without losing any health. Sure, health is helpful against bosses and other enemies you can't sneak up on, but it is undoubtedly less useful than it is for other classes where health is always relevant.
And an assassin will mostly be attacking with single backstabs. Stamina is hardly relevant in this case as you will never run out of stamina in a battle. Meanwhile, you may use a bow, and zooming uses stamina, but that's mostly a convenience and otherwise bows don't use stamina. That really just leaves having stamina for carrying capacity and to sprint for longer. Those are worthy conveniences, but really just conveniences and nothing more.
So what is an assassin REALLY losing by putting some points in magicka. I would argue that the vast majority of the time, an assassin's health and stamina are totally irrelevant.