into an artifact heavy and mission/npc based account. What I mean is no PvP. Not worry about mining at all. Research will be focused on planet structures for a while. Missions. What missions strings should I focus on?

Get the missions done that give you stuff..

Aeon Moon Tier. Controlling the Sithilon Tier. Work on an end-game race, like Lazuli or Talth.

for the npc part
Get the extra npc slots from zolazin chain and lazuli chain
Get the exotic algorithm ability

Silthion elites optional.

You will need billions and billions to upgrade your planets to +12
~3-4 Billion for each planet upgraded to +12s and 1200 def/att structures

I just converted all my planets over to obviator shields and it did indeed cost billions. A painful amount of billions. Be prepared to shell out big time, especially if you have vault planets.

I personally went a slightly more eclectic route. I've done the whole zolazin chain. Same with protogrid. Worked my way up to Contact Coordinates. Just finished unlocking Silthion npc's. I'm also close to being finished the CTP mission chain that gives you scanners, buildings, etc. for ctp. Lastly I'm also half way through the Mylarai chain.

I like to have a little bit of everything based on how effective it is in game. (I also did the TO mission chain up to the point where I got all the shield modules.)

So find what you think is particularly efficient and just have fun.