The idea is that we really only need to kick Ganymede out of Jupiter's Orbit. This can be done by slowing it down to around 6 km/s.

There is really a lot to unpack here.

If you pick Ganymede at its current orbit, and reduce its orbital speed from 10.880 km/s to 6 km/s, it's orbit will become lower (and possibly very excentric, depending on how you meant what you said). This will not kick it out of Jupiter's orbit, but it will mess with the Jovian system. This has the potential to throw some moons off of it, but in random directions and not very far from Jupiter's orbit. On a second pass they will be thrown into random new orbits around the sun.

But most probably you meant to slow down Ganymede (and only Ganymede) on its path around the sun, so that its perihelion is within Venus's orbit. Consider the following:

1. The Δv cost to cause Ganymede to escape Jupiter, then put it in the same orbital plane as Venus, then cause it to intersect with Venus may be one or two orders of magnitude greater than the cost to do an orbital insertion around Venus.
2. An orbital insertion means that the object being inserted will have the same average orbital speed around the Sun as the planet it's being inserted around. So Venus will have to give Ganymede a lot of Δv if it is to capture it naturally. This will take many passes, which might take millions of years (specially since most passes will not result in a flyby).
3. On any pass Ganymede might be thrown off the intended path by Jupiter, Mars or Earth, spoiling your project.

Just decelerate Ganymede on your own on the first flyby. If you wish to see the logistics for this, just send a vessel from Jool to Eve in Kerbal Space Program (Jool is the game's equivalent to Jupiter, and Eve is the equivalent to Venus). Then consider that Ganymede is orders of magnitude more massive than a spaceship, but then again, if you are moving it you are a Kardashev level 2 or 3 civilization anyway.

The idea is that we really only need to kick Ganymede out of Jupiter's Orbit. This can be done by slowing it down to around 6 km/s.

There is really a lot to unpack here.

If you pick Ganymede at its current orbit, and reduce its orbital speed from 10.880 km/s to 6 km/s, it's orbit will become lower (and possibly very excentric, depending on how you meant what you said). This will not kick it out of Jupiter's orbit, but it will mess with the Jovian system. This has the potential to throw some moons off of it, but in random directions and not very far from Jupiter's orbit. On a second pass they will be thrown into random new orbits around the sun.

But most probably you meant to slow down Ganymede (and only Ganymede) on its path around the sun, so that its perihelion is within Venus's orbit. Consider the following:

1. The Δv cost to cause Ganymede to escape Jupiter, then put it in the same orbital plane as Venus, then cause it to intersect with Venus, all together, may be one or two orders of magnitude greater than the Δv cost for the last thing left to do: an orbital insertion around Venus.
2. An orbital insertion means that the object being inserted will have the same average orbital speed around the Sun as the planet it's being inserted around. So Venus will have to give Ganymede a lot of Δv if it is to capture it naturally. This will take many passes, which might take millions of years (specially since most passes will not result in a flyby).
3. On any pass Ganymede might be thrown off the intended path by Jupiter, Mars or Earth, spoiling your project.

Just decelerate Ganymede on your own on the first flyby. If you wish to see the logistics for this, just send a vessel from Jool to Eve in Kerbal Space Program (Jool is the game's equivalent to Jupiter, and Eve is the equivalent to Venus). Then consider that Ganymede is orders of magnitude more massive than a spaceship, but then again, if you are moving it you are a Kardashev level 2 or 3 civilization anyway.