I haven't had any real issue moving multi-hundred-million ton habitats. It will take a few years to move them (i remember building one that was making 57 km/s with a 100kt civilian tug) but a few years is nothing, it just makes it into a project that you have to wait a short while for its completion.
I really hate having to build massive tugs, it just doesn't feel right - what I wish was the ability to have multiple tugs tug the tuggee simultaneously so I could instead built more smaller tugs.
Yes, shame we cannot have multiple tugs that tug one huge target, as is done in real life too. Something like a command where a tug join another tug and from then on they act as one until you separate them again. It would probably require too much work that could be better spent elsewhere.
Also no offence, but that one module is practically useless if you do not have tens of such ships you are not terraforming anything in any given playthrough (well...maybe if you get a superclose to ideal planet, then you just might do it in a few decades).
Simple mixed cargo/colony would be much better idea imo.