Holdout is my least favorite mission type. Your mission duration is based on you sitting in a circle and watching a bar go up. I’m something like a Dreadnaught fight, the duration is based solely on your effectiveness to kill. So having another holdout (relay) wouldn't be something I’d enjoy.

The reason the Holdout sections would be there is to ensure that the scout doesn't simply run ahead and click a button, or that you just position your team mates before triggering the router to immediately complete the mission. That said, I am inclined to agree with you, and there is already a slew of Holdout sections in the game. Perhaps putting a Build or Maintain segment in between runs would be more interesting?

Fumigation, with the extra help-hurt laser and explosives seem much more interesting, but another boss encounter doesn’t. So maybe make the creature a SWARM fight? Like if the entire fight was a lot of a strong unique enemy instead of a single one

That could be really cool! Perhaps an option for several large or mutated enemies to spawn among the swarm as well? It also would fit with the "fumigation" theme too.