1. You need less DeltaV to launch from an orbital position, not more.
2. Why waste time, resources, and energy constructing a space elevator when you can much more rapidly land on planet and start with a powered, central hub to civilization? The concern of the arkships is the survival of humanity on fertile worlds, not to make a rapid transition to spacefaring as easy as eating cake.
3. Nuclear reactors powering thrusters are not emitting an appreciable amount of radiation unless they are damaged. There are also none in existence that can approach any reasonable percentage of lightspeed.
3b. The thrusters are unknown (to us) tech, just like how to get kyrium. It's handwaving for story as well as intentionally limiting our tech for gameplay reasons.
If you want a space elevator for the sake of a space elevator, fine. I think that could be cool too, even though it's rendered obsolete by the tech level of the game (SSTO craft presumably being the norm). I also think it would be more fun to build one that to just have it sitting there from the beginning.
But the way you frame your argument like you are right and the devs are wrong, without accurate or compelling reasoning, makes it hard to take seriously.