Expanded is the best format for discard based decks. Night March and Vespiquen/Flareon are the classics, Archies Blastoise variants are definitely still playable and Maxies gets weaker without Archeops but Gallade is good enough that people will still run it.
I personally love playing expanded Rainbow Road. It uses a Battle Compressor/Ho-oh-EX/Darkrai-GX/Energy Switch draw engine. Basically compress your Ho-oh/Darkrai and a bunch of different types of energy into discard, when you pull them out they bring energy with them which you switch onto your Xerneas-BKT to attack. The list I play uses 8 different pokemon so it definitely has a tool box feel to it, can also go for Jolteon lock with it.
If you're looking to make a discard engine running the GX pokemon you mentioned I'm sorry to say life is going to be rather rough for you, you need to build around one or two pokemon for the most part to make a reliable deck engine, and the deck you mention here runs at least 3 different energies, possibly 4.
Marshadow is really a card designed for expanded and Japanese standard, it's not very good in international standard as it stands because it lacks things like focus sash and battle compressors. Darkrai works as a pump to bring energy out of discard in Darkrai Turbo, it's much better in expanded where you can battle compressor it and 2 darkness energy into discard, attach 1 with ressurection, 1 with a dark patch, then 1 from hand to hit 130 on turn 1.
Ho-oh-GX is probably the most stabdard-viable of the ones you've listed, people usually don't play it for the GX attack but rather play it in Volcanion as a Kiawe target. The best target for the GX attack is Mega Charizard Y. Ultimtely it still plays too slow for what the game is atm and can be punished too easily.
