Paul’s letters do not date themselves internally pre-70 CE, apart from the Aretas allusion understood to be Aretas IV mentioned above. The reason the letters considered genuine of Paul ARE firmly dated pre-70 by ca. 100% of New Testament scholars is paradoxically on the basis of texts which many of the same scholars openly concede are not reliable for history: namely the Gospels and Acts.
In the end, the argument for the letters of Paul as post-70, apart from plausibility and making better sense closer to the time of publication of the collection as you noted, is its consistency with a picture in which Christian origins is arguably better understood as emerging out of and in the aftermath of the First Revolt.
My problem with this point is that the two thieves detail are part and parcel of the same anti-marcionite polemic behind the titulus crucis, identifying univocally the principal victim as the Jewish Messiah. When at contrary for Marcion the true Jesus was the "Jesus Son of Father" (parodied by the Judaizers as 'Jesus Bar-Abbas').
So, if the Joseph of Arimatea is Josephus and the two thieves are derived from the Josephian episode described above, then the point that is going to be made is always anti-marcionite and therefore not original of the Earliest Gospel Passion Story, that included only the following episode :
“Are you the king of the Jews?” asked Pilate.
“You have said so,” Jesus replied.
[Pilate] had Jesus flogged, and handed him over to be crucified.