Earth and Thraxa were marked for invasion but the idea is that he could take his sweet time if he wanted too. They mostly wanted Nolan to see if he could find a way to create more Viltrumites, and they’re never in a hurry for the most part. Nolan had a huge window to do it, but when Mark got his powers, suddenly the window was gone, and he had to do it immediately. If Nolan lived on Earth and managed to have child and he didn’t tell them or he died, they would call him a traitor/failure and kill him anyways. He’s supposed to get the planet ready for them, and they wouldn’t really give a shit if Nolan died in battle because they just think “well he’s dead, so he’s a failure, and not worthy of our respect” but they find sneaky shit like poisoning him in his sleep as unacceptable, and would unleash brutal retribution.