I think that Matthew had access to Mark's account when he was writing his, but also his own independent sources to add to it, and that Luke had access to the writings of both Mark and Matthew, along with his own sources.
Whether or not the writer of John’s Gospel had access to the three synoptics may be a matter for scholars. He doesn't seem to make much use of them however. There is evidence for an earlier origin of John’s gospel, perhaps earlier than Mark.
John reached its final form around AD 90–110, although it contains signs of origins dating back to AD 70 and possibly even earlier.
Wiki: Gospel of John
Again this is a matter for experts. But it’s clear they do accept the Q, L and M sources, as well as independent sources for John and Acts.
So the other two synoptic gospels did not simply take off on Mark and add their own invented embellishments. Not denying some 'poetic liberty' may exist. The lack of direct eyewitness testimony, perhaps or perhaps not, cannot be used to just dismiss the validity of the gospels and the NT