To me, this looks like a sort of final exam for Tua. They've stacked the offense. Assuming the OL improves (and it would be a virtual impossibility for it not to do so), Tua should be able to lead that offense to some real improvement. And, if he can't, the Dolphins will have 2 firsts to maneuver with if they want to grab a QB in next year's draft.
As for Mayfield, there's not really a reason for any team to trade for him. His money's guaranteed, so let the Browns pay it. The Browns are going to have to cut Mayfield if they can't trade him, and there are only a scant handful of teams where Mayfield would have a chance to compete for the starting job, so competition for his services will be limited.