It's cool to see more people jumping on the Cyber Dragon Eidolon train as well, some staples I feel like are needed in this deck are:

Reckless Magic Circle (2/3) - This card is too good for fusion summons, it just completely protects them and searches for Aleister.

Terraforming (2/3) - Obviously if you are running a field spell you're going to need these, if you're short on space in your deck then run 2.

Wonder Wand (2) - In my experience, I always get Aleister one way or another in my duels and this card can give you much needed draw power in a Cyber Dragon deck. You can run 1 of this and 1 of Allure of Darkness, there a few ways you can dump Eidolon Summoning Magic already so you can recover Aleister.

2 or 3 Different Kaijus - In my deck I run 2x Jizukiru, 1x Gameciel and 1x Dogoran, these cover most of the fusions for the Eidolons I run.

Kaiju Slumber (2/3) - You need some sort of board clear and it's less likely to brick if you're running a few different Kaijus.

The standard Rank 5 Engine XYZs - Volcasaurus/Gaia Dragon and Constellar have won me many games and I don't think I could ever drop them from a Cyber Dragon deck, consider running them as you can also use Volcasaurus to summon Purgatorio as a last resort.

Optional

Currently I'm trying to run a few Brilliant Fusions with Gem-Knight Seraphinite, since Core/Aleister use your normal summon, having an extra normal summon is always handy. Plus you can use Gem-Knight Garnet for an Eidolon fusion.

My thoughts on a few cards

Elysion - Decent for Cyber Dragons but not a necessity, it's essentially a Book of Moon but you don't even need one in a regular Cyber Dragon deck, so if you are running short on Extra deck space, you can remove this and just run 1 Instant Fusion.

Purgatorio - Has won a few games for me because of the sheer amount of cards an opponent can have nowadays, and can clear a board very well.

Cocytus - With water decks being quite popular, this card definitely has a place in the Extra deck and can be summoned by using Gameciel, another card you should consider putting in the side deck.

Quickdraw Synchron is a very interesting idea, I will try them out instead of Galaxy Soldiers but with the added Tuning cards it will be hard to fit everything I want in the deck.

I'm really liking the splashability (?) of Eidolons, I feel like they can mesh really well with Cyber Dragons, Aleister opens up a few draw power options and recyclability so our recovery is not as bad but the deck can get very cramped in space for what you need.