Personally I think the Diamonds kept Lapis’s away from combat because they had the potential to be a threat to the Diamonds.
Like our Lapis was arguably the most powerful gem in the show after the Diamonds, she managed to face off against the Crystal Gems while she was cracked, was the only non fusion other than Steven to be able to beat Jasper on her own, and was able to restrain Blue Diamond. And that was just one Lapis on her own! Imagine if they’d been trained to fight and then had decided to rebel? A Diamons could handle an army of Quartz like organic life could not, but a team of Lapis’s? That would be a massive threat!
But they can’t be powered down and still efficiently terraform entire planets, so I think they were instead designed and kept in conditions where they wouldn’t rebel (judging by Blue’s surprise at seeing Lapis I think it was deliberate). They were given what were, for gems, a very privileged and pleasant life. Meanwhile I think their character flaws of being natural a little anti social, self centred and mean, were an intentional way of keeping them away from other gems; especially as the un-traumatised ones we met considered themselves other gems, or using their powers beyond their intended purpose (and from the way Lapis acted I think that’s another pretty universal Lapis way of being).
Youre right! They must have given potentially powerful gems jobs that would distract them from ever discovering their true potential like peridots metal bending :0.
I have also noticed the Lapis Lazulis we’ve met have a teenager vibe centered around them. Maybe its a dominant trait such as the rubies being angry, and pearls being elegant.