You bring up valid points.
Now that I think about it, Magearna EX does have it's drawbacks. The most common deck that runs Espeon EX is Drampa GX/Garbodor, and Magearna is useless if Garbotoxin is active. With that said, there was a Solgaleo GX deck that went far recently in either a Regional or League Cup that ran one copy of Magearna. I saved the deck list, but can't recall who played it or where it was played.
I can see the benefit of Registeel, for sure. A non-GX Basic that's able to accelerate Energy is important. I would probably remove the Magearna EX from my list and add a Registeel.
I get your reasoning for not minding losing a 2-energy Cosmoem to Guzma, but that's still not a trade-off I'd want to make. Imagine having the Solgaleo GX and third Energy in your hand and losing that Cosmoem. You have plenty of other options for getting Energy in the discard. I'd rather just Rare Candy into the Solgaleo GX and not have to worry.
I think a lot of this is one big "if" scenario. It's good to think about every possible scenario, but it's also good to not get too caught up in it. There's an infinite number of scenarios that can happen. At the end of the day, as long as your can provide valid reasoning for what you choose, there's no wrong answer!
Are you refering to the Solgaleo GX/Rayquaza build? I'm not sure if it was running any Metagross, but if it was it would justify Magearna EX. I consider it to be an OK tech in lists that utilize Metagross GX since Geotech system and having an attack that doesn't force discard means that it doesn't alter your momentum while providing protection. And when you Max Potion a Metagross GX to discard its energy and Geotech it back, it's at full HP and out of Espeon EX devolve KO range. The system runs a lot more fluidly than Magearna EX in Solgaleo.
As for the Cosmoem/Guzma situation, the point is that I would have multiple Cosmoem on turn 2 because I have 12 ways to see it, so I still have my turn 3 Solgaleo.
Turn 1: Open Nebby, Lele, Brigette for Nebby, Nebby, Zorua. Attach energy to most appropriate position (usually bench to force Guzma, but sometimes active to Dust Gathering if going second.)
Turn 2: 4 Evosoda, 4 Ultra Ball, 4 Cosmoem. 12 odds of seeing them and I need at least 2 to have 0 risk of being slowed down. Odds are, I will see 1 in my current hand and then Sycamore or N for the second one (relaying my experience from playing Salazzle GX/Luranits/Octillery that I mentioned before). Just like Rare Candy build, turn 2 is the turn that decides the success of my deck. If 2 Cosmoem get in play, I am on pace to perform as expected. If not, then the game is in my opponent's control. If 2 Cosmoem are in play, you cannot stop me from a turn 3 Solgaleo GX. Under certain circumstances, it may prove to be more advantageous to go for 1 Cosmoem and 1 Zoroark GX, who may possibly draw me into my 2nd Cosmoem as well as apply pressure.
And from there it looks pretty similar to rare candy build.
Also, I keep neglecting to explain why I'm not a fan of turn 2 Sol Burst GX. I don't like that play for the simple reason that you are drying up your own energy resources. I believe the optimal Sol Burst GX would be 2 more metals onto active SuMo Solgaleo GX who already has 1 metal to activate the GX attack, 1 DCE onto an upcoming New Solgaleo GX, and then 2 more metal onto a wild card Cosmog/Cosmoem which is only 1 energy attach away if you go for another SuMo Solgaleo. Running 9 metal energy and 3 DCE, this means that, at best, you have 4 metal and 2 DCE remaining in deck. On turn 2, you still have a very large deck of around 40 cards, and unlike Metagross decks, you can't operate exclusively out of your discard pile under any circumstances. You NEED to draw energy and attach for the 3rd one for future Sunsteel Strikes, or blow an extra turn on energy acceleration, in which case it would be more like energy decceleration since you're slowing yourself down.
Just from my own statistics, I almost always prize 1 basic energy, so I could be extra picky and say you will have 3 metal and 2 DCE left in deck and you need to see metal ASAP. You have 1 Sunsteel Strike ready to go next turn, and maybe you'll be lucky and get your prized energy from your first KO, then the turn after you Newgaleo to accelerate from discard. If you didn't see any energy, you're putting those on the one that already has 2 energy on it for a 4 energy Sunsteel Strike later. But instead of accelerate with Newgaleo GX on this turn, you would much rather attach the 3rd energy naturally and do back to back Sunsteel Strikes. If you succeed this, you have 2 prizes to go, but both of your SuMo Solgaleo GX are now dried up. Is that okay? It might be okay because Newgaleo GX can clean up.
But how likely are you to draw 1 of your 4 or less remaining metal energy in 2 turns? Are you willing to waste a DCE on Sunsteel Strike if you see it instead to satisfy 2 metal 1 colorless cost? What if you brick on draw supporters? Do you have the 2nd Choice Band you need to OHKO another Solgaleo GX/Decidueye GX too? Are you going to drop a Choice Band on Newgaleo GX instead to 2 hit KO anything bigger than 240 HP? You can't Prominence GX to keep your Newgaleo GX alive in the front lines either since you already GX'd. If it falls, you have 4 prizes to go, 4 or less metal energy in deck, and no access to your discard pile for at least 1 more turn. That SUCKS.
This is the scenario that I imagine in my head and is the main reason why I don't understand why people are so gungho about rushing Sol Burst GX. Please explain to me if I'm missing a certain aspect of the GX attack or if my energy distribution ratios are crap. I know you are not forced to attach all 5 energy cards. If you already have energy in play, you don't need to go for all 5. But I don't even know if it would be worth it to GX attack just for 2 or 3 energy. If you need to buy more time, Prominence GX can buy you more time to see a few energy cards.
There's a lot to respond to so I'm simply just going to keep talking until I feel like I've addressed things everyone has said.
As for getting put behind, my goal in this deck would be to have staggering evolutions. By this I mean there will often be points in the game where I have all 3 lines of evolution in play at the same time - 1 Solgaleo, 1 Cosmoem, and 1 Cosmog. Losing a Cosmoem is always the worst case scenario, and yes I absolutely have no access to late game or mid game instant recovery. This is a problem that realistically can't be ignored, but I would also like to argue that it might not be getting ignored at all.
Zoroark GX is not a win condition for even my deck list, but it's a "don't lose" condition. It's not going to have as much impact as a rare candy SuMo Solgaleo GX coming in for 230 damage or a Newgaleo GX accelerating 2 energy onto a soon to be SuMo Solgaleo GX, but it does have the power to stand alone for solid 2 hit KOs and 1 turn evolution. If you take out my Cosmoem, fine I'll make a Zoroark GX instead and apply pressure that way while I slowly recover my Solgaleo evolution line up. Heck, if I make Zoroark GX turn 2 and start applying pressure on your board,
do you have time to worry about KOing my Cosmoems at all, especially if I didn't invest any energy to them? I haven't thought about it that far yet myself. If Zoroark GX gets the first swing in on a Newgaleo GX and I commit a choice band to solidify the 2 hit KO, let's just say that things become very awkward for both players. The big question for my deck is the following:
Is Zoroark GX a strong enough recovery to nullify the need for instant rare Newgaleo GX? Both can 2 hit KO one another and both need only DCE to apply pressure. Arguably, getting out a Zoroark GX is far easier than a rare candy Newgaleo GX at any turn in the game, and that advantage alone may or may not be a game deciding factor. This is where playtesting comes in.
Ultimately, the point of running Zoroark GX is that delaying my Solgaleo line up does not guarantee my loss. Manual evolution is enabled by simply introducing a Stage 1 in the deck with enough presence and no energy conflicts, and Zoroark GX is the best solution.
As far as discarding with trade is concerned, I run plenty of 4 ofs that I can make an educated decision to discard. Generally speaking, Pokemon are the next best target since the rescue stretcher can actually search those back out for you. The extra 1/1 Zoroark GX line is also a viable discard target once you already have one in play. Also, in line with what Houchins mentioned about the Cosmoem/Guzma scenario, my goal is to have 2 Cosmoem in play so I'm guaranteed 1 Solgaleo GX, and it'll be that Solgaleo GX with a DCE who accelerates the energy I lost to a KO, not a Registeel. It usually won't be Registeel by virtue of you absolutely have to set up Newgaleo GX.
I feel like I placed too much emphasis on Registeel. It's there because it's just plain good. It's a good alternative method of energy acceleration
if you lose a Newgaleo GX you have an immediate recovery option with enough HP to survive some bullying. And if I open it, I get to bully off any starters (like a Cosmog) and get an early start to Registeel/Zoroark GX energy combo. Lastly, it's a free drop for satisfying 120 damage on Zoroark GX. It has many good uses but it's not a win condition. I just feel that its versatility deems it worthy of being a 1 of. I honestly would like 2 in my deck, but as mentioned before it's not a win condition, so I would not sacrifice any win condition cards to play more.
As for the Volcanion match up, while it doesn't stand much of a chance either, Zoroark GX at least isn't a free, effortless OHKO for fire. Ho-Oh's need choice bands, but most builds are running FFB, and Kiawe, and Turtonator needs steam up no matter what to pop you. It's still not optimal, but at the very least I can still have something on turn 2 that will put the heat on fire. That was intended, and you are allowed to facepalm.
I don't feel like I covered everything but I also think I talk too much, so let's call this a post for now.
EDIT: An idea came to mind -1 Evosoda for +1 Wally. Wally allows for turn 2 Solgaleo GX to become a thing, and with Zoroark GX around, I still can get some chummy draw power even if I use him as my supporter for turn. It doesn't have to be early either. I can be in the middle or the late game. In this circumstance in particular, I feel that Wally is better than rare candy. I only need 1 of him, and through Lele + Ultra can see him a lot more often than trying to squeeze in 2 or 3 copies of rare candy to the list. Alternatively, if I prize the turn 1 Brigette, I can still turn 1 Wally to have turn 2 Solgaleo GX via Evosoda.
Worth considering, I think.