A friend of mine notes that Agatha has already come close to breaking through a second time, and it almost killed her ("forty-five point three seconds from exploding"). So apparently part of the difficulty in going Super-Saiyan becoming a Queen is surviving the process.

They also note that when taking down Lucrezia-in-Agatha, Otila sneered at her as a coward.

Put together, their theory is that Lucrezia wants to become an immortal God-Queen, but doesn't want to risk her own precious hide. So she's looking for alternatives...