You can't "just" do anything. Landing a significant payload on an asteroid is barely do-able right now. We are a long way from building mining rigs and railguns up there; clamping on an engine and some extra propellant is much more viable in the near term - generously estimated at within the next 30 years.
The point is that if each return journey requires propellant, you need to restock. An EM rail gun can just use solar power for continuous return. Rail guns are a present day technology.
You will already need substantial solar power and batteries, that’s not in question.
No-one is talking about multiple round trips to the asteroid. Moving the whole asteroid at once to a more mine-able location (the bottom of a gravity well, where people can actually use the material) is what I expect to be easier near term. Mass-driving from the asteroid to a target location is interesting too, but seems even more sci-fi. Frankly I don't expect to see either happen in my lifetime.
You're right, efficiency won't come first - but you're replying to a comment about efficient means. I'm 65plus, 30 yrs doesn't seem so very long to me.
