But Paul was contemporary and had met with witnesses to the resurrection to discuss matters of orthodoxy.
Possibly, although we cannot be certain who met whom. But why does this have any relevance to his purported encounter with the post-resurrected Jesus.
They would have been familiar with his account and able accept or deny Paul's claims of witness to the resurrection.
But we have nothing from those people - we only know their claims etc via third parties, for example the (unknown) authors of the gospels and possibly Paul.
But actually this doesn't help your cause. Paul's claimed vision seems to be just that - a vision. Indeed there seems to be a complete disinterest in a physically resurrected Jesus from Paul. So if this early claim reflects those of the disciples etc then we are dealing not with any sort of physical resurrection at all. Merely some sort of visionary experiences, which is very different from a physical resurrection.
The notion of a physical resurrection seems to come along much later - nothing from Paul, nothing from the earliest gospel Mark (in its non-doctored form), only in the later gospels.
So, actually we see an evolution over time - from the 'resurrection' being simply a visionary experience largely amongst the disciples and close contacts of Jesus (plus the 'me too, look at me, look at me' Paul) through to the much later claim of a physical resurrection.
The interesting historical question isn't whether there was a resurrection (that is a faith claim not a historical claim), but why the early church felt that they needed to morph the story from something which isn't particularly super-natural (a vision/dream of a dead person - happens all the time) to a supernatural physical resurrection.
An obvious answer is that once the narrative had moved beyond those there at the time a mere vision/dream experienced by someone else didn't seem significant enough and the classic exaggeration and hyperbole kicks in this people feeling the need to ramp up the claims to make them more impressive. Also the audience - I think there is an argument that once the early church had moved beyond attracting jews (which had largely failed) that to attract non jewish audiences some more significant spectacular claims were needed.