“Terraforming Mars might be the best option for leaving earth but something like that would take quite a long time.”
Not if the machines are self building and dismantling...
Basically imagine sending one robot to a planet that starts to mine, refine and construct other robots from the raw materials in that planet’s soil.
Then various specialized robots fill the planet and alter the composition of the soil and the atmosphere until the ideal environment is achieved. Then they dismantle themselves into useful structures for humans, start growing food and we begin to arrive!
Sure that’s the way we’d go about it, but it would still take quite some time for the new plantlife to generate enough atmosphere to make it a suitable environment for Earth.
At minimum, multiple milleniums I would think.