I note you omitted to mention the theory which is by far the simplest explantion of the synoptic primacy theory (it is not a “problem”) and the only one in accordance with the testimony of ancient sources: Matthew wrote first, then Luke (who no doubt had access to Matthew as well as other sources such as the BVM’s recollections), then Mark was written as the record of a series of sermons which St Peter gave in Rome, quoting both Matthew and Luke (as a way of approving the use of Luke in the West) and adding a few oof his own reminiscences and remarks, with the final chapter of Mark being written in Alexandria after St Peter’s death at about the time of which St Mark moved to Alexandria as its first bishop (presumably having been sent there by St peter both to spread the faith and to ensure orthjodoxy and to avoid his suffereing the same fate as Sts Peter and Pauil as Nero’s persecution raged in Rome).