Hey, I think you're definitely on the right track for the deck. I already played the deck a bunch with all the cards revealed so far, and I think that's all we need in regards to cards for the deck, but here are some things to consider: You always need to be setting up Solgaleo GXs, so you can't afford to miss Evolutions, and if you do, you definitely can't rely on Rare Candy alone for catching you back up. For that reason, I was playing 3 Cosmoem and 4 Rare Candy. I was also playing a Cyrus and a Choice Band.
Cyrus makes your opponent choose only 2 of their Pokemon to keep, and because you should be OHKO-ing anything in the game, you destroy the setup they have. I was previously playing a Parallel City before this card was revealed, if that information matters to anyone. The one-of Choice Band is amazing, absolutely amazing. It means that Sunsteel Strike OHKOs literally anything in the game, and if you really need to, Solgaleo Prism does 190, but mainly Sunsteel Strike. And in a format where Solgaleo is everywhere, which it will be, this WILL matter. Just not enough to warrant 4 copies. That's just overboard. Drop the RayRay. I dislike Registeel, but I can't ignore the points brought forth, so that one is fine. But Rayquaza does the exact same thing as Newgaleo only it does 30 as opposed to 120. It doesn't have the HP to have any backbone either, so it really just takes space in the deck, and on the bench. I suppose 1 argument would be that it finishes what Sunsteel Strike started, which is the reason I tested with it, but if you're using the attack, use the one that does 120 and won't be OHKO'd. And no Max Potion? Max Potion is the closest reason to making this deck borderline unbeatable.
3 Choice Band is the minimum that I would do. We're still in a Garbodor format, and we are definitely going to be seeing Garbodor try to shut down this deck, which means field blowers are going to be everywhere and only 1 choice band is not going to stick it for you for all of your KOs. I also don't see how you will continue your OHKOs if your SuMo Solgaleo GX with Choice Band gets KO'd - you now have no way to OHKO anymore Solgaleo GX's. Not sure how you're consistently grabbing Choice Band when you only have 1 copy as well. I can drop 1 Choice Band for 1 Cyrus, but I'm still not all that convinced by him yet to be honest. I think any player can play around Cyrus as long as they expect it to be in someone's deck. Cyrus is also really bad if your opponent has 2 Leles on the bench - you just give them a free refresh of Leles back into their deck for more supporter consistency. At the very least you are almost always going to be shuffling 1 Lele back into the deck. That Lele could help your opponent grab a game winning Guzma. Solgaleo doesn't need a lot of set up to function well. Your optimal board after taking a Cyrus is a SuMo Solgaleo GX and a Newgaleo GX with a Cosmoem in play ready to take the place of whichever of your Solgaleo's get KO'd and you really don't need more than that. I would rather drop 1 band to add the 3rd Cosmoem since you bring up a good point.
Rayquaza is here because not every game is going to be perfect and you need utility in the form of basics and not stage 2s for those bad games. I really don't care if it gets KO'd because it just forces the 7 prize game. SuMo Solgaleo GX and a Rayquaza functionally does the same thing as a SuMo Solgaleo GX and a Newgaleo GX when you consider the mirror match and the fact that everything is dieing in one hit. I don't care about the difference in damage because this is an OHKO deck. Whether you hit 30 or 120 with your energy acceleration attack is almost irrelivent when the next turn you're going to be hitting for 230 or 260. As long as Rayquaza can get me 2 energy accelerated onto a SuMo Solgaleo GX before it falls, even just once, that's all I need it for. It's a basic that requires no set up. Drop it down, attach a DCE, Ultra Road it into the active, and you don't lose any momentum.
No max potion because Solgaleo GX is the very definition of an OHKO format. I really see no point when Sunsteel can hit for 260 and DM Necrozma GX can hit for 250. Max Potion isn't going to help much in an OHKO format. You have Prominence GX to handle any spread damage or poke damage from decks that won't be grabbing OHKOs. And yes, I would absolutely purposefully save my GX attack for Prominence GX if I knew I was up against some kind of spread damage or 2 hit KO deck that can't scale over me. If I'm facing a Solgaleo GX deck, I would Sol Burst GX 9 out of 10 times. But if I was up against Glaceon GX who really likes poking me, I would set up Newgaleo GX and just take OHKOs and Prominence GX to heal off all the spread damage if I felt threatened by Espeon EX devolve KOs.
I'm not sure what decks you've been playing against with your build, but the only 3 decks I'm worried about right now post SM5 are Salazzle GX/Garbodor, some kind of Glaceon GX variant, and Newgaleo GX. This deck list I made is expecting that those 3 decks will be the most popular in the meta, and stuff like Metal Silvally and Golisopod variants and Gardevoir GX will die off in the wake of these new decks. If I'm wrong then my list will change. This still really is just a skeleton list based on speculation, predictions, and what I think will be good.
At the end of the day, the deck list that you take to an event will always be decided on meta calls and personal playstyle, so this deck list can never ever truly be complete. It's just a complete skeleton that I think includes the baseline of what the deck needs to function.
What I will say though is that this deck takes a hard auto loss to Salazzle GX/Garbodor right now. Part of the purpose of running Zoroark GX was to counteract the possibility of Salazzle GX/Garbodor. Remember, Newgaleo GX's ability is a defensive ability, meaning it works on your opponent's turn and not your turn. The reason why you need field blower is to enable Ultra Road on your turn and nothing else. Even if you field blower on your turn, your opponent can still reattach a tool on their turn and Salazzle GX choice banded will slaughter you since the ability will be shut off again. Zoroark GX gives you something that can eat Salazzle GX in the early game while you set up your OHKOs and his trade energy discard outlet is stupidly useful. This is another reason why you need Rayquaza. You need a 1 prize energy acceleration option that doesn't get OHKO'd by Salazzle GX's 110 damage attack. KOing Garbodors is going to be something this deck wants to do in order to keep Salazzle GX away.
Whether or not Salazzle/Garbodor sees much play depends on how much Glaceon GX sees play. This post is already too long though so I'm just going to stop here.
