Time to crank out that spreadsheet and notice it got waaaaaay less complicated since right before the Link era. This deck aims to combo off so if you're no into long combos that keep on going, it's not the deck for you. If you are a fan of utilising all summoning methods (except for Ritual), like to build boards that are actually Nibiru proof ( thanks to D/D/D Sky King Zeus Ragnarok ) and want to show your opponent the true terror of capitalism, boy, do I have a deck for you.
D/D/D usually aims to fill the board with big monsters without going for too many generic Extra Deck options which might not seem that good but this deck has tons of great in-archetype endboard pieces that still are threatening. We have multiple negates in our Extra deck and the side deck offers some weird floodgate options as well, just because. Now as for the non-engine: You can play everything your heart desires in the slots occupied by normal monsters and this list is the absolute minimum for the deck to still work well (16 non-engine is quite a lot in my eyes)
As for some other engine questions that might come up:
"Why only 1 D/D Savant Kepler?" Droll & Lock Bird exists
"Why no Nightmare Throne?" Same as above but runs into Droll even worse since it only searches Kepler or D/D Savant Copernicus
"Why no D/D Count Surveyor? Count Surveyor is just worse Dark Spirit's Mastery by requiring a D/D discard (quite a steep cost) and being a Monster which are more easily interrupted than Spells
"Why so many D/D Scale Surveyor?" Freeza looking guy is just a good extender turning any other D/D into an instant D/D/D Abyss King Gilgamesh or a Rank4 if you got the requirements down
"What is Go! - D/D/D Divine Zero King Rage doing in here?" Bro's chilling (and either forcing hand traps or stopping them altogether). Fun fact: The way the third effect applies to Trap cards that can be activated directly from the hand is still ambiguous in the TCG but they definitely cannot be used in the OCG. Clarification on that will hopefully happen post DOOD TCG release with the deck being back in competitive circles and Dominus traps running rampant.
"Why is Dark Contract with the Witch in this list?" Mostly as a way to turn on Siegfried against a board breaker like Dark Ruler No More or Forbidden Droplet. If you can't chain with monsters directly then you just gotta chain a Trap to get your monster effect back online for that chain. The 1k stat boost during the opponent's turn is also just funny and the pop is decent removal if you still have the resources for it.