I have yet to build a post rotation Greninja BREAK deck myself, but from the few pre-rotation decks I've faced off against, once they got to Frogadier in the active it was kind of all over for me (Alolan Ninetales GX)
That being said, the first idea that comes to mind is to put Evosoda in the deck replacing Dive Ball. If you Evosoda one Froakie up, then you all of a sudden have all of your available Frogadier on the bench. I'm honestly not sure if speed, consistency, or damage output are the main concerns for Greninja BREAK.
As far as much ups are concerned, I don't think Shadow Stitching is nearly as good against Gardevoir GX as it is against Metagross GX. Without Geotech, Metagross is looking at 3 manual attaches to even swing at all, and it has no way to OHKO Greninja BREAK without Kukui even at that. It also can't rely on retreat/max potion to discard 3, Geotech back on 3 to get back to back Giga Hammers. Once Gardevoir GX gets 4 or 5 energy, it can just keep swinging for OHKOs with no attack restriction on Greninja/BREAK every turn, assuming you have the 1 energy to shadow stitch, and if you don't then Gardevoir GX will just secret spring OHKO something every turn. The fact that Gardy GX can utilize DCE and Metagross GX can't is also something that works more in Gardy's favor.
Basically, unless Greninja BREAK can find a way to consistently take down Gardevoir GX, or at least go 50/50 with it, then I don't see it being one of the best decks of the meta. But I do consider it to be an advantageous match up against Metagross, Alolan Ninetales, and Volcanion.
I haven't researched the deck that much myself, so perhaps a more knowledgeable player can fill you in.