The Quran states explicitly that the messenger's duty was only "to convey the message (29:18)," and he said nothing on his own as his own sayings (69:44).
It states that the message that the messenger conveyed was the Quran only (42:52 & 14:52 & 69:44).
Therefore, to follow God's words in the Quran
would be to follow the messenger, (4:80), as the words of the Quran is the messenger's speech (69:40).
It also claims to be the Qawl or the speech of the messenger (69:40).
The Quran claims that it contains answers to ALL relevant questions (25:33) and contains the best explanation (Tafseer) of itself (25:33 & 2:159).
The Quran claims to be the Hukm or commandments of God, according to which humankind is to be judged (5:48).
It also states that it is the law/way with which God sent the messenger (45:18 & 42:13).
Who would know best on how to talk to humankind but their creator?
Therefore, it makes no sense to say that outside sources better explain God's word.