You have two good options in my opinion:

1) Remain with CO and find the best second choice Star Alliance partner, I'd recommend Air Canada for their lower elite threshold and better product than UA or US.

2) Switch to AS and have a second choice Star Alliance partner. AS has most non-stop flights from SEA, along with CO, DL, NW, AA, BA, QS, etc. as FFP partners and unlimited elite upgrades coupled with the third best domestic coach product after Co and WN. The question to ask yourself is how much time you want to spend changing planes.