Two billion years from now I’m pretty sure that all kinds of exotic energy such as fusion, Zero-Point energy, antimatter, and other things we haven’t even dreamed of may be available by then. Only focusing on the type of energy available today is ridiculous unless the entire population of the planet is going to go to sleep for the next two billion years.
Additionally you have ignored another possible solution. Refueling the sun. The sun will only enlarge and produce these negative effects if it runs out of fuel. It’s primary fuel is hydrogen, the most simple and basic element in the universe. I would think that in one or two billion years we should be able to artificially create almost any element, most certainly the lighter ones like hydrogen. Resupply for the sun’s fuel shouldn’t be a serious problem, then the other problems go away. Got to think big and outside of the box if you are going to think two billion years into the future.