You made some good observations. It's really hard to know for sure why Paul did not mention women as eyewitnesses. In fact, he did not mention the empty tomb at all. It could be that the pre-Pauline evangelists knew about but intentionally excluded women's testimonies. But if that was the case, it entailed that the empty tomb was well known in that early stage of Christian movement. That would make the silence on the empty tomb weird in that pre-Pauline gospel message.

On the other h and, to say that the tradition of the women's testimonies had not been invented at that time is also a conjecture. There is no way to be sure.

Nevertheless, I would advise against placing too much weight on the multiple attestations across the four Gospels. They are not four independent sources after all. Matthew and Luke show signs of familiarity with Mark or they share common sources with Mark. It is not impossible that the empty tomb narratives in all four Gospels can be traced back to only one oral or written source.