You can use regular fission as a dense energy source. If you are worried about the waste, you can just blow it up. In space there's no fallout. The particles from the explosion will just move radially forever. Most of them (99.9999..%, too many nines to count) will keep moving for billions of years through empty space without encountering anything. Oh, and as a curiosity, if you blow up a nuke in space, there's no fireball. The fireball we see in movies of nuke detonations are due to the air absorbing the X-rays from the nuke and becoming overheated plasma. But there's no air in space, so a nuke explosion is invisible and silent.
Layered mirrors concentrating monochromatic solar irradiation into laser cavities, and beaming power to where needed, is the fallback until aneutronic fusion works.
But, yes, it would be pointless to try terraforming Venus without von Neumann machines, and then it would be thick with those, instead.