[credit_guy]

That's not how people intend to colonize Mars. Nobody is thinking of building rockets on Mars for a very long time. A few decades, maybe a century or more. The idea is to only refuel the rocket on Mars. You land the rocket (which comes from Earth) using aerobraking and retroburns. You refuel there. You take off. The only thing that gets manufactured on Mars is the fuel (technically the propellant).

On Venus, you need to find a way to not land the rocket, but to make it stop gently next to the balloon, attach it to the balloon somehow, adjust the buoyancy of the balloon when this happens, so the balloon does not suddenly drop by 20 km where the pressure difference, and additional heat, can destroy it. Then you need to manufacture the propellant inside the balloon, then you need to find a way to make the rocket fire its engines and take off, but without destroying the balloon in the process, and you need to adjust the buoyancy of the balloon in reverse. Of all the steps, I'll grant that the manufacturing of fuel might be roughly as easy as on Mars (the Sabatier process and all that). But the rest have no parallel on Mars, and don't seem possible, no matter what technology we come up with.

[lmm]

Landing/recovering aircraft on an airship is proven technology, and hooking a tether onto something as it falls through a buoyant atmosphere is quite possibly easier than stopping a rocket precisely at ground level where any screwup will turn it into a crater immediately (as is the case for Mars). Yes the balloon needs to be able to adjust its buoyancy to maintain stability but that was always a requirement; sinking into the atmosphere is a relatively slow process as long as your balloon is big enough. Likewise air-launched rockets are already proven technology - you can either just let the rocket drop away far enough for clearance before lighting its engines, or if you're feeling fancy then the rocket uses its own sacrificial balloon to rise as high as possible in the atmosphere before launch (of course that means you have to be able to manufacture balloon fabric or arriving rockets have to bring their own until you reach that stage, but that doesn't seem insurmountable).