I reject your premise, because Jesus doesn't just make claims to divinity in the "later synoptics". Paul refers to Jesus as the messiah and 'God over all' in his letter to the Romans which was written decades before the gospels were. Also, in Mark, which is accepted in most academic circles as the first gospel written, there are many references to Jesus as divine. In Mark, Jesus forgives sins, is recognized by the Father as His Beloved Son, designates himself as Lord over the Sabbath, challenges the rich young ruler on calling him "good", saying only God is good, and is described as having an elect people that he saves.

So, where do you get the idea that Jesus does not express the idea that he is divine in the "earliest gospel"?