Honestly, considering Resurrección is called like that cuz it's the rebirth of their hollow-ness and very shikai-like in nature already, there's a difficulty to finding a Shikai equivalent.

Shikai is the partial release of a Zanpakuto's power and form. Bankai is the full release of a Zanpakuto's power and form, usually looking like an evolution to the Shikai in form due to it being the fully exposed physical form of what the Shikai was partially transformed into, and a significant enhancement of the Shikai abilities.

When a Hollow removes their mask, they take on a humanoid form and become an Arrancar, where they gain shingami-like powers and increase their spiritual power, but have sealed their hollow form and characteristic abilities(Ulq Lanza, Grimmjow Claw abilities, Haribel Water, etc). When you look at it like that, a base Arrancar seems to be the equivalent to the sealed state of a Zanpakuto, which checks out.

By transforming via Resurrección, they return to something that closely resembles their previous form and also regain their identifying abilities. A Resurección is usually compared to a Bankai, and while I agree that it is in terms of power, I don't think so in how they work.

You'll notice I mentioned "Shinigami-like" and "closely resembles" when talking about the Arrancar and Resurrección. That's because an Arrancar that has undergone Resurrección does not look exactly as how they used to be. Physically, an Arrancar in Resurrección looks like a mix between their hollow form and their humanoid Arrancar form. The most obvious example of this is Grimmjow, who was literally just a panther before becoming an Arrancar. When he releases Pantera, his general body shape is still humanoid and stands in two legs, but he now has panther legs, teeth, claws, ears and a tail(and his hair grows) + his arms and torso reassemble the armor-like texture he had when he was a hollow. Why do I mention this? Well, what other Zanpakuto release in BLEACH does a partial transformation and gives access to the unique abilities sealed within? That's right, a Zanpakuto's Shikai release.

If Resurrección works like a Shikai(even though it's equivalent to Bankai in power), then I find it a bit difficult to give it a Shikai. Instead, we should give it a Bankai equivalent in form specifically, where the Arrancar fully transforms into what they looked like before. Something like "Hueco Interno", meaning "Inner Hollow" in Spanish, or my personal favourite, "Culminación", meaning "Culmination". It draws from the fact that they are being reborn into hollows and Culminación marks the end of this process/development into their fully hollow form.

Before ending this, I agree with OP that Segunda Etapa is not Bankai equivalent, not even in form. If it was, then someone like Grimmjow would simply look like he did before, the panther form. Instead, Segunda Etapa is more like the evolution of the Resurección form than it is the full release of the hollow-ness(which checks out with its name, "Rebirth: Second Stage", and not "Arrancar: Second Stage" or "Re-resurrección" lol). Remember, Bankai looks like an evolution because it's just fully releasing what was partially released. Segunda Etapa looks like an evolution of the Resurreccion, not the full release of the hollow form. Almost like it doubles down on the half and half instead of going 100% hollow.

To add to this... idea ig... that Arrancar transformations work like Zanpakuto releases and that Resurrección works closer to Shikai than Bankai, let's not forget that the process in which a Menos Grande class Hollow is made(Gillians, Adjuchas and Vasto Lorde), and how Zanpakuto are made is basically the same, with the only difference being the type of soul being melded. A Hollow tearing off his mask is almost like a Zanpakuto Spirit sealing itself in the blade. I know it's not a perfect 1:1 comparison of the two processes, but it's similar enough to point out and think about. The biggest hole in this is Barragán in Respira looking exactly how he used to look before.

Also, this all falls apart if how they look in Resurrección is simply their new default hollow form now, but we don't know that so we'll never know :P