The waters for when exactly the Diamonds decided to destroy Earth have gotten muddy. We’ve always known that the Cluster took a long time to develop, and that the Diamonds had the power to wipe the planet clean of rebellious Gems, so we assumed that the Cluster was only conceived of after Pink’s shattering and the attack was intended to keep the Cluster safe. After Your Mother and Mine, however, things are less clear. It now looks like the decision to destroy Earth was made long before Pink’s shattering, and without her knowledge.
When discussing the Cluster in When it Rains, Peridot says that she knows about the Cluster from reading a few hundred years of reports. These reports covered the why of the Cluster project, that the rebellion had removed Earth’s viability as a colony, and the how, experiments in the forced fusion of gem shards.
Since we know the project was only started after the decision to terminate Earth was made, and that the project lasted for hundreds of years, we know that the rebellion raged for a long time while Earth’s fate was already sealed by Homeworld. It was Pink’s colony, so one would think that no decision would be made to destroy the planet without her knowledge.
The control panel in the secret room in the Prime Kindergarten, however, bears a yellow diamond—not a pink one. If Pink was aware of the project, wouldn’t it make more sense for it to bear her symbol?
Then there’s Garnet’s story in Your Mother and Mine. Whatever inaccuracies her tale may have had, we can trust her to know about events which affected her directly. The way she tells the story, the Diamond attack came very quickly after Pink Diamond was shattered by Rose Quartz. There was no indication that Homeworld fought them for any length of time afterwards, just that the Diamonds were furious.
Lapis’s story to Steven in Same Old World and Centipeetle’s drawings in Monster Reunion both match up with this. Both describe a panicked rush to get off of the planet, with not all Homeworld Gems making it off in time. The Cluster project was secret, but that doesn’t mean the evacuation necessarily had to be disorderly. If the attack was launched by seething Diamonds, however, it would definitely make sense that Homeworld forces had only a very short window of time before they purged the planet.
Based on Garnet’s story it sounds like that’s what had happened: the Diamonds reacted immediately and harshly to Pink’s shattering, ending the war while their subordinates scrambled to get off-world before it happened. If that’s the case though then the Cluster must have already been implanted before Pink Diamond was shattered.
That doesn’t necessarily have to have been the case, but it’s very likely. Homeworld Gems working on the Cluster project were operating on the planet for hundreds of years, collecting gem shards from battle sites and taking them back to the Prime Kindergarten to test, then to the Beta Kindergarten to create and implant the Cluster. If they only started after the Diamonds purged the planet then they would have had no reason to act stealthily, since as far as they knew all other Gems had been wiped off the face of the planet.
In all that time they probably would have left signs of their presence, if they didn’t accidentally encounter the Crystal Gems outright. Again, they’d be making no signs to hide that they’d been there, so any gem shards that they dug up would be obvious to anyone revisiting the old battlefield.
Of course improbable things happen all of the time, but secretly designing and constructing the Cluster on Earth without even realizing that it needs to be kept secret would be quite the coincidence. If that unlikely event didn’t occur then the only other possibility is that Yellow Diamond made the decision to destroy the planet back when Pink was still alive. Based on the dream in Jungle Moon it would be quite reasonable for Yellow to take an “it’s for your own good” stance with Pink, maybe even throwing in an “if you deserved the planet you would have been able to keep it” for good measure.