To a certain extent, it will not matter how many phones DA has owned and used since the murder of DM. Everyone I know, including myself, does the same thing when getting a new iPhone: we transfer everything from the old phone to the new one. Apple makes this very easy. Even if DA is a tech klutz, either someone she knows or an Apple staffer or an iPhone seller could do it for her. She does love her Geek Squad.
What I don’t know is how much info is lost during such a transfer. For example, if DA deleted texts in 2014 or 2016 (or whenever) and later got a new phone and did a complete transfer, I suspect (but don’t know) that evidence of such deletions would not be on the new phone. Would her iCloud data maintain a record of that? I don’t know that either.
Just knowing what DA used her phone to do will be helpful to LE. (Ditto for her computer, which LE should get ASAP, but that may have literally been thrown away long ago, or perhaps only shortly before her attempted flight from justice.)
If she used any native iPhone apps such as Calendar and Notes, LE can subpoena Apple for the info. LE can also subpoena third party companies that produce apps she downloaded onto her phone.
Of course the biggest problem is that some of the juiciest info will be a decade old, and many companies may not retain data that long. And we know that some apps, notably WhatsApp, don’t independently keep the user data that LE would want.
We already know how reckless DA and her crime family were in planning the murder and comically talking about it using their utterly transparent code. I’m sure there are still some incriminating things on the phone LE has seized from DA. I hope some of it enlarges the pile of evidence against WA.