This is my third Endymion deck. I honestly just like posting it on here every time I update my deck.
I started using Chronograph Sorcerer and Timegazer Magician. I can not believe I did not use them before. Chronograph is a free spell for counters, and makes Timegazer a free special summon (I think this might have been common knowledge but keep in mind I started playing Yugioh like late-December of 2020). Not only that, but if you have a second Chronograph in hand, and you pop Chronograph to bring out Timegazer, then that activates Chronograph's effect to special summon itself from your hand, as well as any other monster from your hand that you want to special summon. That's an instant Crowley for the spellbook engine and you haven't even normal summoned yet (also if you want to keep 'em there, Timegazer has low atk/def values but acts as additional protection for your scales). If you can activate instant fusion to get out Millennium-Eyes Restrict, or activate Servant of Endymion to bring out Jackal King before you link into Crowley then you are basically free to run to grab more counters and also thin out your deck a bit more.
There is also one copy of Super Polymerization. From what I understand, most people say to run 3 or none at all, but honestly with this you are drawing into your deck so much that you either get your ideal board of negates or you draw into Super Poly anyways. This is here because I am tired of going against a first turn Dragoon with a mediocre hand. This way, I usually either have a solid hand that can play around Dragoon's negate and then finish my combo to get accesscode or something and just walk over Dragoon anyways, OR I'll have Super Poly and just take him out for Starving Venom.
Speaking of the extra deck, I took out Day-breaker and Bagooska for Starving Venom and Mudragon. I usually will always try to go for Starving Venom, especially against monsters like Dragoon and Borrelend, but if can't, Mudragon is a good way to interrupt plays as well, especially because if I activate its effect to prevent the targeting of a specific attribute, I can actually stop a surprising amount of combos (though usually if they are able to, they will just go into battle phase and walk over your Mudragon-- it still makes them waste a battle phase and saves you from a potential OTK that turn). I also added in Underworld Goddess of the Closed World from Blazing Vortex. This is also just an amazing card to out boss monsters that "can't be destroyed by card effects/battle and can't be targeted" (Dragoon/Borrelend/etc.), and she has a pretty nice effect too.
Also just wanted to put it out there because I sometimes forget: but if the opp. has monsters like Barrier Statue of the Stormwinds or Vanity's Ruler, you can use Archfiend Eccentrick to destroy them.