The Stackexhangerians live on a habitable, earth-like planet near Alpha Centauri. Problem is that their world is dying. Let's say it was attacked by yet another race of hostile aliens, and most of their civilization annihilated. But some of the population managed to escape in generational ships that don't have FTL travel. They went into cryosleep and it took them 500 years to reach the outer limits of the Solar System.
To make things more "logical": the aliens have more advanced tech than humans, yes, but nothing that would allow to them to take on Earth head-on as they are VASTLY outnumbered. They are also tired and worn out from their travels, and they don't have a lot of military personnel to begin with. Also, this is a near-future Earth wracked by climate change, destroyed resources, overpopulation etc. There is no space for these aliens to land on Earth and grow their own population. They don't have the means to land on Mars or the Jovian/Saturnian moons and "start from scratch": they need a ready-made world. And since they can expect resistance, they decide to soften up the Earth first to make colonization/conquest easier.
How do the Stackers tackle this issue? I was considering two options:
They send spies to Earth and over the years, the spies not only learn about human tech and human biolology, they also indoctrinate disenchanted humans to their side, and together, they develop a biological super-weapon. the bio-weapon decimates most life, but some people prove immune. Now all the Stackers need to do is put boots on the ground and pick off the remaining survivors
They use their last remaining weapons to either bust up Earth, or push an asteroid into it, or something similarly destructive. They will then try their luck at, once again, picking off the survivors using their advanced technology
Do any of these make sense or is there another way they can approach this?