Excellent summation of the Vice Principals' dynamic. I hadn't thought of it in exactly those terms, but whereas Kenny Powers could generally only be so spiteful or depraved before butting against someone (relatively) more normal, Gamby and Russell really do perfect a vacuum of mutually-destructive excess.
I can certainly appreciate why you didn't get into Gemstones, but would recommend giving it another try if the feeling strikes. It's definitely more McBride's show than Hill's, and the whole mega-church millionaires angle immediately removes it far further from our reality than the other characters we've mentioned. Even Kenny Powers was principally a legend in his own mind. But for what it is, I adore it without reservation.
Not sure how much you got to see of Goggins' Uncle Baby Billy, but he's a truly outrageous presence that somehow transcends the memes (though "Misbehavin'" is a true earworm) and reveals himself to be both a deeply recognizable human being and a whirlwind agent of chaos. He's consistently being pushed away from the main family (often with good reason), which in turn calcifies his resentment, causing him to goad the others to their worst extremes.
Additionally, Edi Patterson -- who also does a share of the series' writing -- delivers a lengthy monologue near the end of the season (in an Outback Steakhouse, you'll know the one) that's breathtakingly funny and existentially horrifying, while remaining believable and totally consistent with her character. For me, big swings like that and dropping an emotional, full-episode prequel in the middle of the season (somewhat like the also great Mythic Quest did this year) balance the less-grounded bombast. Different strokes of course, but that's my two cents.