Even if we could somehow give Venus a breathable atmosphere and some water, I'm not sure it would be desirable real estate due to it's rotation. Venus has a very slow retrograde rotation of 243 Earth days, which is longer than it's year of 224.7 Earth days. I understand this unusual circumstance gives it an apparent day of 116.75 Earth days, meaning the Sun rises in the west on Venus and sets in the east nearly four months later.
If Venus had an Earth-like atmosphere what would the temperature extremes be like? Super frigid at night and boiling hot at noon? Or would constant powerful winds/convection currents mitigate the extremes? If this were some other world with these conditions would there be lifeforms adapted to a four month day/summer and a four month night/winter?
And while there has been much speculation on how we might terraform the atmosphere I've never heard of any ideas on how we could get it to spin faster.
The ideas of terraforming Venus were built around the idea of propting a big continuous precipitation of the chemicals in the atmosphere - called "the big rain" according to the article in the link.
I recall in the 1980's it was thought that terraforming Venus might be easier than mars because of the substractive nature. But one could land and presumably survive on Mars without a terraforming first step, so I think that people think Mars might indeed be a little easier in general, and settleable immediately.
Alright, what about terraforming via microbe, a more mundane version of The Chthorr Wars concept.
We seed the upper clouds with CO2 munching bioengineered algae/bacteria, create an oxygenation event and break down the pressure cooker cloudbase?
There is a short sf story that brings that up. I think a scientist wrote it.
Basically a space station orbits above the clouds and seeds the Venusian atmosphere with reducing bacteria. There are some blocks they have to resolve, but it isn't with the science.
The other documentary I saw said that the Martian core was solidified, and it would need to be restarted to get a magnetic field back. Well beyond our tech level.
I remember another documentary on the same subject which rejected the idea of starting the core of Mars as really too hard. Instead it was recommended to put a large (~10 Tesla) magnetic field generator in the L1 orbit position. This is both strong enough to match the Earth's, and well within our current technology capacity.