Frankly I'd say the main reason why there's no need for Sylveon GX is because the new non-GX Gardevoir power creeps it.
Judge is not being played often enough and at high enough counts per deck for it to discourage Magical Ribbon. Magical Ribbon was still the best way to set up at the end of Gardevoir's last season even when N was being played at 4. If my opponent goes out of their way to Lele for a Judge just to brick themselves on 4 cards, then it's still a win for me. I can just Magical Ribbon/Brilliant Search again next turn anyway. Either way, the fact that we now have a 1 prize attacker who hits harder than Sylveon and can also search just as well as Sylveon means that Sylveon is obsoleted. Plea GX also loses value since most decks nowadays are Basic/Stage 1 focused and Parallel City has rotated. Nuking people's benches just doesn't really work anymore. Alolan Vulpix is a much better basic than Eevee, too.
I think the Meganium is interesting spicy tech, but also don't think your bench has room for it when the goal of the deck is to have as many Gardevoir in play as possible. I'd say this deck's "optimal bench line up" is 2 Gardevoir GX, 1 Gardevoir non-GX, 1 Alolan Ninetales GX, 1 Tapu Lele GX (you're going to use one most games), and either Oranguru, Alolan Vulpix to Beacon, or the 4th Gardevoir (either GX or non-GX depending on situation). In games where you aren't going to be using Alolan Ninetale's GX attack, you will likely end up Twilight GXing for resources anyway, including your rare candies. Twilight GX is a GX attack that I feel a lot of people underutilize. I almost always use it after I get a KO and know my opponent has no way to counter KO because it means I will always win the resource battle against my opponent. Restocking energy and rare candy after getting KO'd is a huge plus as well.
But that choice is up to you. I haven't playtested the new Magenium in Gardevoir and it might end up being a crucial partner.
In my opinion, I would play this deck top heavy.
-1 Kirlia
+1 non-GX Gardevoir
With the addition of Alolan Ninetales GX, you should be rare candy evolving your Gardevoir almost exclusively. Since Espeon EX is out of standard and no one is playing Shining Jirachi for good reason, you don't have to worry about devolve KOs preveting you from accessing your Stage 2s anymore. You can keep 1 Kirlia for when things go poorly, but frankly I would play every Stage 2 deck with 0 Stage 1s with Alolan Ninetales GX in the picture. It's been very successful in all of my playtesting with Stage 2 decks like Solgaleo so far. I would even say get rid of the 2nd Kirlia for a 3rd Alolan Vulpix. Since this deck is ability based for its set up, you can afford to attack with Beacon to search for your Pokemon, or rather you might as well attack with Beacon to optimize your turns during set up. Having a 3rd copy also means that the one sitting up front to Beacon can afford to get KO'd since you will have enough copies with or without Rescue Stretcher.
Just my thoughts.