It's unknown whether it is capable of protecting the human cargo on its way to Mars. Maybe a design with habitation encased in the propellant tanks is a better idea for longer transits.
In any case, Starship is well positioned for Earth and Moon travels, as well as for sending equipment to LEO and the Moon.
With autonomous robots perhaps it could be done in a span of million years. Maybe. A relatable analogue might be removing liquid from a glass with a tiny straw. Yet I'd think it is easier to remove, than to add material when speaking about terraforming.
https://sciencing.com/albedo-planets-5203.html
https://www.universetoday.com/36833/albedo-of-venus/
Also, I doubt an insignificant chunk of ice (relatively speaking) could help cool it, or increase albedo.
I think the first order of business on Venus, if one wanted to colonize the surface (rather than colonize the atmosphere, which I think is doable), would be to "harvest" the gases of the atmosphere and thin it out to something more resembling Earth's.
Also, you are probably looking more to continent or small moon size quantities of ice (if that even works, and if the magnetosphere issue was somehow addressed) that metropolis.
Second, the Venusian atmosphere is oppressive as hell, you'll not be solving that problem anytime soon, as we can't even solve Earth's much easier atmospheric GHG problem.
Martian colonization will be tough. 30% gravity, 3% atmospheric pressure, radiation, very cold, very dry... however, building a base is conceivable and we think we know how to do it.