Until I got to maybe Heroic Iron Juggernaut, I sat at about 12.5K Spirit with the legendary Meta gem. The proc from that, using Omen of Clarity when it procs and popping Innervate at around 80% should keep you in plenty mana. Popping your mushroom often is also a good way to be mana efficient. I'm sitting at 1 570 item level now but still only have around 14K, and that's just so I can overheal a lot on things like Heroic Malkorok. 14.5K at 545 just seems like a lot.
SotF with Wild Growth is definitely the goto AoE. If things start looking pinchy, (especially if people are stacked, but even if not) you can pop Nature's Vigil and Rejuv spam, hitting Genesis if damage is getting high. I love that trick on farm, because the damage output of single target healing spells through Nature's Vigil is pretty significant.
My default for Symbiosis is a Shaman for Spiritwalker's Grace. A Mage or a Hunter can give you Ice Block or Deterrence for cheesing bombs on Spoils, for example. Tanks are viable if they need an extra cooldown.
Sounds like you have a good grasp of the class. I too used to raid Disc, but due to token balance in my raid and the spammy nature of Disc I went Druid. Haven't looked back.
Good luck