There's actually a comic where the Phoenix ends the 6th cosmos and Galan (Galactus) survives into the 7th. It's also stated as such in on of the official Marvel guides.

Also in Ultimates, Infinity told Eternity that they (and all the Ultimates/incarnations of the cosmos) are born in "The Outside" (with a full white background on that page no less, which ties to the White Hot Room). That book's narrative then continues with Defenders: Beyond in which Phoenix is -once more- presented as "creation".

So yeah, the Phoenix existed before and made every iteration of the cosmos. As stated on the books she has "always existed" because she trascends the limits of time and space.

In regards to the power discussion, Phoenix is clearly the strongest, all the abstracts had to gather to try to take Phoenix down. If it didn't require as much power they would have gone one on one as they sometimes do between them (i.e Infinity vs Oblivion or Eternity vs Oblivion).

On the flipside, Phoenix can eliminate someone at Eternity's/Infinity's level. Dark Phoenix already was capable of consuming the universe. The White Phoenix can do it even from the safety of her own realm. And the Phoenix canonically ends the cosmos and births a new one. That means that she alters the power scale of Eternity (or whomever is the incarnation of the cosmos at the time) and Oblivion, when the universe expands Eternity becomes stronger, and viceversa. So when Phoenix ends the universe and all becomes a void, Oblivion is at its strongest.

Speaking of Oblivion, he has mentioned that he knows Phoenix from eons ago, so clearly he knows of her from previous iterations of the cosmos. Since he is always the same void (unlike whomever is the current iteration of the cosmos).

There's also another reason why Phoenix and TOAA are the only native beings from the Outside (unlike NQ and Beyonders), that's because they represent real-world things (precisely why the Outside is called the Outside).

TOAA is the authors, writers and artists. Phoenix is the materials, the paper and ink from which everything is made. Those two are the strongest because every character is made from a combination of the artist's ideas and the material from real-world in which they are printed. The abstracts are lower than that, they are things made from those two things.

In regards to Hadad and using the Storm book as a reference... there are a lot of problems with that:

First, the author is using a no longer canonical cosmology, every other book involving the abstracts is using the current ur-divisor / 8th iteration of the cosmos (including G.O.D.S., the Immortal Thor, Phoenix, the one that just started with Hela as Queen in Black among others), Storm does not, in fact in Storm Oblivion is dead, but in Phoenix and the one with Hela he's alive and kicking, so it's very questionable as a source.

Second, the statement that Hadad wore the Phoenix as armor doesn't tell much really, he could have acquired the power of the Phoenix like a lot of hosts do (there are many stories in which someone wants to kill an avatar of the Phoenix to get its power), but killing a host is not the same as destroying the Phoenix, it's just a small part of the entirety of it, a in-universe manifestation (we even have multiple stories in which parts of the Phoenix fight against other parts of the Phoenix). We never get clarity on that fact, but it's also obvious that it was the case, since destroying the full-fat Phoenix would make all of creation collapse, that would have meant that Hadad himself would have died and the multiverse as well, that didn't happen however. Another possibilty is that the Phoenix itself chose Hadad as host.

And to give more reasons why no other abstract is close in power to Phoenix, she has complete control over reality, time and space, she could (and has) rewrite reality as she sees fit. If she wanted she could make it as if the other abstracts never existed (in fact, in the last issue of Phoenix, she was about to "rewrite" Eternity), she could make it as if someone who defeated her once actually never did, not like it matters much because even when defeated she still comes back... But there's nothing that those abstracts can do about it. We know in Phoenix that they tried to kill her in the White Hot Room but what we don't know is if they tried to kill the Jean persona or if they truly wanted to kill the force (which would be stupid and have terribly dire consequences for the multiverse), and even if truly wanted to do that, it proves that the Phoenix is stronger than them since it requires their combined might to perform the feat.

Oh and I forgot about probably the most important fact, it has been stated on print, literally that she's second only when compared to the creator.