The thing is that you're making it sound like being stuck on Earth is a hinderance when the opposite has been true in the past. I mean, Raditz has been fighting in space for decades against warriors from other worlds and a bunch of humans on Earth past him in less than a year by doing basic training. And for the record, Kami didn't really do anything with the humans cast, something he laughed about when asked if he gave Goku special training. The best they got specifically from Kami was ki training, which doesn't necessarily effect strength.
And the 'best method' has been shown to be the Hyperbolic Time Chamber where the results varies. Again, the humans got stronger than Raditz just being doing basic training and Raditz also had the entire universe. And as I pointed out, 17 can train for years straight without stopping when even Frieza would eventually need a water break.
On an unrelated note since I don't want to double post, I figured out why Merged Zamasu comes off as lackluster power wise in the manga to me, and it isn't just him throwing bricks. Since everyone is already dead in the manga, there is no real panic when seeing his power. Part of what made Merged Zamasu so powerful in the anime, despite his destruction being fairly limited, was that you had the people on Earth being stuck in ground zero and having no where to hide from this monster. They're force above ground where they can be vaporized by a random energy bolt and risked being buried alive if they stayed underground. We also see people around the world basically looking at the end of days. You feel for the humans because things were horrible before and now it just got worse. It doesn't help that despite Merged Zamasu's power, everyone comes off as chilled except Trunks.