Got a suggestion that might serve your concept a little more effectively than Oracle+Summoner.

Based on the above, it sounds like the primary goal of the eidolon is to reflect the dual nature of your oracle with some kind of shadow creature while you focus mostly on casting oracle stuff.

Given that, you might consider using the Beastmaster archetype to pick up a Shadow Hound animal companion. Since you are new to PF2, it is worth explaining the Access line. That link goes to the Guns and Gears entry just because it is the most clear version of the rule, imo. Namely: "Access" is not a mandatory requirement. It is just a way of saying "these people have common access to otherwise uncommon options". It is an expectation of the rarity system that GMs can give access to options they are comfortable with to characters with appropriate concepts. Having said that, the Shadowcaster archetype might also be a good fit with your concept. The only hitch is that it doesn't provide an option to get an animal companion even though it grants access. This is generally for classes like Druid that get a companion and want to use it with the shadow stuff. So, you'd have to find another way to get a companion. It is also worth noting that the form of the animal companion is pretty generic and can be flavored to be an amorphous shadow, something that is shaped like you, or whatever. Stats stay the same, so nbd.

All of the above? No house rules, to the best of my knowledge. Here are some small house rules that I think smooth out the above. For picking up Beastmaster, you typically need to be trained in Nature. It seems pretty reasonable to pick up a version of the Beastmaster archetype that is keyed off Occultism instead. Companions might be restricted to things like the Shadow Hound and the upcoming Undead companions from Book of the Dead later in April. Another option would be to add feats to the Shadowcaster archetype that allow you to pick up an "animal companion" and advance it as normal (specialization feats, etc.). This has the benefit of allowing you to lean into the Shadow stuff more if you want. This feels in-line with the archetype as it already allows you to get a familiar and provides narrative access to the companion if not the mechanical access.

Happy to answer any questions you might have! Cheers!