In Zoroark Break, you would rather go into the break 9/10 times because Foul Play is jacked. The problem with Zoro GX is that its damage is too low. There's no reason to go into GX instead of break. And while there is synergy with Darkrai GX, you're putting a card on your bench that won't do anything. Even if you attack with it, you're not doing any real damage. You can't use his GX attack, and there's no point in slowing down Zoroark Break just to add techs that aren't going to help your win condition. And Zoroark GX as it's own deck is just too weak.Zoroark GX can also be mixed with regular Zoroark BREAK assuming that it doesn't rotate and there's crazy synergy between Mallow/ Zoroark GX/ and Darkrai GX. The only thing I see missing from that deck is a good way to inflict status so as to make proper use of Darkrai's GX attack.
I honestly don't agree with your A Muk GX assessment. With a strong line of Salazzle, cards like Devo/ Accerola (Po Town)/ and Super Scoop Up I really don't think consistency is an issue, especially considering Brigette. Hell, A Grimer from BUS can put another Grimer on the bench, so it's pretty hard to have a non-start. DCC isn't really a high attack cost, and it's got a free GX attack that is perfect for stalling and forcing out a switching option. I don't understand how it's got damage issues... It's hitting 1hkos on most of the meta with Salazzle alone. Teching in Raichu for Metagross and Decidueye will easily put you over the top. It's retreat costs aren't really concerning to me considering Guzma/ Olypmia/ Escape Rope/ Switch/ Float Stone are all still in the format and will probably remain there post rotation. One thing that won't stick arond but that will play off that high retreat cost very well though is Heavy Ball.
Post rotation will be slower anyway. I see two main deck types. The fast T2 attackers that get two shots on everything but are weak to prize denial, and the big, slow, one shot machines.
Muk has consistency issues. For example, in order to get the status every turn, you need to have the Devo or evolve another Salazzle. At 110 hp, Salazzle is an easy pick, you have no way to search it outside of ultra ball, which you have four of. Yes, Grimer search Grimer is a thing, but again, retreat cost. You can get Guzma stalled so easy. Play escape rope, play switch, play float stone, that's just more cards that slow you down and hurt you against Garb. And as far as the OHKO on things, the meta is shifting to 210-250 HP Pokemon. Unless you're going for Tapu-Lele GX, what are you OHKOing? Or another status in, cool, more consistency issues. You have 60 cards to fit two+ evolution lines, energy, draw support, and resetters for your statusers. There's a reason status decks aren't meta, they're to clunky and slow to keep up.