What is the best way to stay competitve in TDM and Co-op against high Cst planes? People who spend a gargantuan amount of money getting all their end-tree planes to Level 20 seem to dominate any TDM matches they play. Should I save up tons of stocked fuel and get some credits contracts and then just do a solid four days of grinding for an end tier plane or should I focus on leveling up other aircraft?
You could always only join or start cst restricted games until you have a built up higher level plane. If you want to maximaze your credit rewards, only spend stocked fuel on weekends during 6x sorties and wait for level 2 or above special raid missions. Then sortie with 6x fuel and a elite mercenary contract.
FWIW, very few people have end tree, lvl 20 planes in my experience. There are a lot of people with 2200+ cst planes, but you don't need to get to lvl 20 to do that. My suggestion would be to pick a plane or two that are at least 700, but not one of the uber expensive superplanes, and use them all of the time so your opening up extra expansion slots while also building them up. Once you get those near lvl 15 or so they'll be about as good as what most of the best players have.
I have a lvl 13 ADF-01 at lvl 13 that's 2300 equppied and a lvl 14 Wyvern that's about 2250 equipped, but you don't need planes that far to the right on the tree. There are plenty of good players using the F-15 S/MTD, any of the 750 fighters, or any number of the special planes that you inevitably stockpile if you play long enough.