And that's exactly my point. If some guy was writing a letter to other churches, there would be no point in going through the effort of referencing peripheral things that both parties were or were not aware of. As you say, it was enough work just to compose what they were trying to communicate. I would not expect a fairly random collection of letters to reinforce each others historical narratives. I wouldn't expect the author of Acts to have randomly referenced Pauline letters for any reason. I mean, those letters weren't even yet collected together as significant, right? How would that come up? "Hey man, you know that other letter some other guy wrote to someone else?" I just wouldn't expect biblical writings to present a coherent narrative. They were not written to be.
Well, the author of Acts did reference supposed letters [plural] from the Jerusalem Church and also did mention the contents of those letters.
And not only did the author of Acts mention the contents of the Jerusalem letters, it is stated that they were given to Paul and his companions and that they delivered them.
Acts of the Apostles, as the name implies, is directly about what the Apostles did after the ascension of their resurrected Jesus.
Saul/Paul is mentioned over 150 times in Acts, far more times that even Jesus and Peter combined, where it is claimed he persecuted believers, blinded by a bright light, carried out miracles, raised the dead, was shipwrecked, imprisoned and preached in synagogues around the Empire.
One act is missing about Saul or Paul from Acts.
The act of writing a letter.
All the so-called Pauline Epistles were fabricated after Acts of the Apostles.