I think there might be a bit of a misunderstanding here. I listed the Shadow Powers for completedness, when actually, my advice is Alchemy. It doesn't use Magicka, so you can spend it on Stamina instead, and adds a whole bunch of other effects, including Frenzy, Healing, and Paralysis. None of these are unique, but they are spread out amongst Illusion, Restoration, and Alteration, use Magicka, and take up a hand to cast, so you can have at best 1 dagger out at the same time. You also can stack these with other effects, if you know any good Recepies. For instance, I like Paralysis with Lingering Damage Health for those real tough guys, so I can just stick them, and leave them to die. (You can even get Damage/Ravage Health on the same Poison, so you put an even bigger dent in with your Backstab.)
This versatility also extends to after you get Shadow Warrior. Huzza! You don't have to make any more Potions of Invisibility! However, you still have all the other effects you can get from your Craft, some of them combining 3 in the same dose! It's the gift that keeps on giving, while Illusion is eventually overShadowed, leaving you with all those Perks, and Magicka points you spent on it. This is not what I, personally call "Perfect" even if you have a definition of "Pure Assassin" which includes Illusion, and Magicka. The only downsides are that you have to replenish your stocks, and stop by the Alchemy Table every once in a while as well as make room in your inventory for all those ingredients. However, in the late game, once you dial in what to pick up, and distil your staples to the minimum you can depend on minus the redundant effects you get from Skills/Perks alone, it's not as much of a burden.
Now, you have to understand, I HATE alchemy. It annoys me more than the other two crafts, and I hate Crafting! However, for this particular build, I rate it above Smithing, and Enchanting to wring those lasts drops of perfomance out. You get most of your damage from Backstab Multipliers, sure you Can get even more from the Grindstone, but for the few hits you actually need it, you can just as easilly pop that potion of Fortify 1 Handed, or Archery, and dip the tip/s in that Poison of Infernal Nastiness you prepared earlier. Quite literally, every single enhancing effect you need for an Assassin? There's a Potion, or Poison for that, or you can make one...