John was still receiving revelation decades after some of the other Apostles had died. Since the synoptics were written 30-50 years before, their content was well known. John filled in what was not in the synoptic Gospels. His purpose was different.
But wait! The early Church was not established or run on a “bible”, a “book”, “scrolls” or any writing. No one searched for the “Q” source. It was the oral Apostolic teaching and preaching. Read the Prologue to Luke (Luke 1:1-4). Theophilus learned nothing - zero, zip, nada - from Luke’s Gospel. Luke wrote it only to confirm the oral Apostolic teaching that Theophilus had already received.
Read the Didache, the earliest non-canon writing extant. It was composed while the Apostle John was still living and preaching. In it, there is not a single mention of any writing of any type. It is the pure Church as Jesus founded it. You do not need a bible or any writing to be a Christian. We are blessed to have the scriptures, they are irreplaceable, but they are not absolutely necessary to the true Church.
Since 16th century European man invented sola scriptura, we are tricked into proof texting, slinging verses and demanding written proof. The primitive Church did none of this. They simply believed and obeyed.