Light sails + fusions engines look good, shoudl be able to get 0.7 - 0.8 c with that combo. I suspect the first trips will be similar to the Mayflower voyage, ie no one expected them to come back.
I think the first extraterrestrial colonisers woudl not expect to return, but set up a permanent colony and stay there.
and how the hell would that help with earth's energy problem exactly?
The better solution long term is we invent a way to get off this planet and spread ourselves across the galaxy. Thats means more technology, not less.