If a rocket exerts 10 lbs of force on a shuttle, or an astronaut does, it makes no difference. The shuttle barely moved because it has a lot of mass. Mass is still relevant in space. The bigger the Dreadnaught is, the more force required to move it, even though any minimal force will technically still move it an equally minimal amount and it won't ever stop. Let's take two of your rockets. I'll put mine on the back of the shuttle, and you put yours on the Dreadnaught. Which one of us is going to go faster? We'll both move infinitely, but I'll be doing it a lot faster than you.