One alternative would be to make diamonds out of the extracted carbon and drop them to the ground. The released oxygen could then bind to crustal aluminum, iron, silicon, calcium, etc.
Venus has sadly little hydrogen, so you won't get oceans.
But, yes, it would be pointless to try terraforming Venus without von Neumann machines, and then it would end up thick with those, instead.
The Kuiper Belt offers more than the inner planets, provided energy is solved e.g. via aneutronic fusion.
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.
There is plenty of un-oxidized material on the ground. Oxygen just needs to be kept off the carbon. And, it needs stirring. Big meteors could do a bit of that.
Earth gets its stirring from tectonics, which Venus lacks.
This occurred in stages.