I think it's typically fun for casual play and pre-releases but otherwise think this isn't too clunky but outright too inefficient for the Pokemon that are run for this strategy.
Charizard GX's Wing Attack in my opinion is bad, Crimson Storm is cool and plausable thanks to Kiawe but discarding 3 energies is a lot to do. 300 damage is also "overkill" so while fun and amazing on first sight I don't think it's worth the effort required. Raging Out GX is a good effect but also quite expensive at 3 energies again... To me Houndoom EX just isn't the type of Pokemon that has what is required these days to be part of any deck at all. Milling 2 is just too slow.
Objectively speaking I think milling as is isn't a competitive option in this format with the sets available. The Charmander to Charizard GX line doesn't speed itself up and doesn't have some in between cool Powers that could assist it (now). If (for example) Charmeleon would mill 2-5 cards upon evolving I'd consider this strategy to be more viable. Now all you can do is present Pokemon who give away 2 prizes and mill. As a result to me Charizard GX is just a very odd Pokemon. If it's GX would say mill 10 and do 10 damage for each Pokemon (or something along those lines) milled I'd see some great potential while now I do not.
The prime issue I have with this is that A. It's bad against Greninja/Alolan Ninetales (GX). B. It's much slower as Volcanion/Kaiwe decks. C. Gardevoir GX/Sylveon GX completely counter this set up while punishing long and energy heavy set ups. If you can't deal with that (what I precieve) as current top I wouldn't play this type of deck.