I didn't use flasks much i just potted mageblood / sage elixir was good for most of the early raid content, I went alchemist because of the mageblood potions from zandalar friendly. but being able to make fire protection potions and your own mana potions is invaluable as a healer especially for your first few raggy attempts. i don't remember flasks being much more powerful than just multi-potting they just lasted for 2hrs and didn't disappear on death. if you don't die then potting was probably better for me, mana regen was a nicer alternative that getting like 70 int or whatever.
I was good with those consumables, 12mp5, 18int/spr, food,mana oils. your not going to need flasks for t1/t2 its just not going to be that worthwhile. a hell of a lot easier to make stacks of regular pots and save the black lotus for when you hit a wall. flask recipies are drops as well so you have to actually farm them first.