Here is a really good reference on the pros and cons of methods to terraform Venus, with some strong physics and science behind them
How Do We Terraform Venus?
The part you are interested in, is perhaps
In contrast, increasing the speed of Venus’s rotation would require
tremendous energy, not to mention a significant number of impactors
that would have to cone from the outer solar System – mainly from the
Kuiper Belt. In all of these cases, a large fleet of spaceships would
be needed to haul the necessary material, and they would need to be
equipped with advanced drive systems that could make the trip in a
reasonable amount of time
And when it comes to removing Venus’ atmosphere, things are equally
challenging. In 1994, James B. Pollack and Carl Sagan conducted
calculations that indicated that an impactor measuring 700 km in
diameter striking Venus at high velocity would less than a thousandth
of the total atmosphere. What’s more, there would be diminishing
returns as the atmosphere’s density decreases, which means thousands
of giant impactors would be needed.
In addition, most of the ejected atmosphere would go into solar orbit
near Venus, and – without further intervention – could be captured by
Venus’s gravitational field and become part of the atmosphere once
again. Removing atmospheric gas using space elevators would be
difficult because the planet’s geostationary orbit lies an impractical
distance above the surface, where removing using mass accelerators
would be time-consuming and very expensive.
It has also been suggested that Venus’ rotational velocity could be
spun-up by either striking the surface with impactors or conducting
close fly-bys using bodies larger than 96.5 km (60 miles) in diameter.
There is also the suggestion of using using mass drivers and dynamic
compression members to generate the rotational force needed to speed
Venus up to the point where it experienced a day-night cycle identical
to Earth’s (see above).
The last one is interesting, because it posits that you do not need to actually HIT Venus, just fly buy it with enough mass.