Some of my thoughts and questions:
-You didn't really list how this job would heal. >x>
-What is the cooldown like on Mix? How frequently are you going to use it versus the individual potions you use to mix with?
-Like RicaRuin said, Phoenix Down is a slightly weaker Healer LB3, which can still turn the tides of a fight too easily. I would suggest just let it be a single-target revive. With this, it can be an MP-free, insta-revive that you used your mix on instead of DPS or Healing.
-What kind of additional effects does having/being a Lab Assistant give?