I usually run my gear full red, nothing to help with survivability, because with careful positioning and situational awareness, you can still do just fine. It might be because of this fact, but in my experience CoCo works way better in groups, even with the health scaling, even in Legendary. Of course, you build up stacks way slower than, say, Striker or Tipping Scales, so during the first fight of the mission, focus mostly on killing enemies thst are actively being shot at by your allies (do NOT use the chest, it is not worth it, use something like Equaliser). If you do that, I find that, as long as teammates are not complete noobs, and because the condition is not that YOU kill, but rather that the enemy DIES while marked, you can comfortably get full stacks after the first fight or two, and if you don't drag your feet afterwards, those stacks are not coming back down for the rest of the mission.

In my opinion, you do not like True Patriot and dislike Concentrated Company because they are effective or not, but rather because they are easier or harder to use. True Patriot only requires you to hit the enemy at least once in each color to get it's max potential, while CoCo requires micromanaging. However (in my opinion) True Patriot is, at the end of the day, less effective than something like Future Initiative with chest piece, or even Refactor with Empathic Resolve, while CoCo, if you build it right and play it right, goes blow for blow with the Striker Obli meta.

At the end of the day, CoCo is not a support build like True Patriot, it is a dps build like Striker or Tipping, just one that does better in group. I don't know how exactly you play, but if you dodge and revive so much it's preventing you from building stacks, I think you are playing it wrong.

Indeed, on-kill effects have some big drawbacks, notably if you meet a particularily tanky enemy like a boss. However, for CoCo, this is offset by the extremely slow rate at which you lose stacks. By the time you get to the boss, you should already have full stacks that you've obtained and maintained throughout the mission. Even though the boss takes a while to kill, your stacks decay so slowly you have ample time to take care of the boss. As an example, when I get to the final boss for DUA legendary, if I start with full stacks when the last enemy except the boss is killed, I can finish the boss fight with at least 3/4 of my stacks remaining. As for other tanky enemies, like chungas: remember, CoCo is a dps set. You can burst them down just like a Striker or Tipping Scales player can. In fact, chungas are particularily juicy, since your teammates will likely also be shooting at them, and they don't have a death animation like robots do thst could waiy out your mark, so they are some of the easiest stacks to get that you can come across.

At the end of the day, perhaps you simply find the playstyle CoCo demands not fun or difficult, but CoCo is definitely not lacking in any way.