It seems very unlikely that the book of Matthew was written by Matthew the tax collector.I find it highly unlikely that Matthew and John actually wrote those Gospels given, as you say, the seeming reliance on Mark and Q.
John's gospel is also a big question mark.
John the disciple was a key member of the twelve and sometimes he is speculated to be the "beloved disciple". Still he was one of those often rebuked by Jesus for lack of understanding. Could he have made the leap to writing the gospel of John with its well-developed theology?