I think you cavil a bit finely when you complain that Jesus “never told a disciple to write.” And so? Jesus also never told a disciple not to engage in buggery, either. What we see in our modern bible is a small but adequate sample of a biography which could easily fill an encyclopedia. In general, “Go and make disciples” would entail writing when that was the medium that made sense. And it turns out that some of those disciples are made through the writings, centuries after the writings. The writings fulfill their purpose.
Of course the gospels do not contain every word Jesus ever spoke. And, of course, Jesus did not repeat EVERY teaching of the Jewish faith when those would never be questioned by his audience, such as the Old Testament’s moral teachings.
My point was that it seems improbable that, if God willed that ONLY writings were to be the source of our belief, that those writings themselves would make no mention of that fact.
The Scriptures themselves (the NT) do not contain any statement of the doctrine of sola scriptura.