I have a 2012 FX4 but it only has 18,000 miles on it. I have used it hard, towing a trailer for work most days, stop n go traffic, and multiple stops during the day. The 6 speed transmission on this vehicle is said to be solid by Ford techs I have seen on YouTube. But I would not call it smooth or sophisticated. Hopefully your situation is "normal" for this vehicle. But owning a European car now and several in the past, I can tell you that a US pickup's shifting will not seem so smooth compared to better more sophisticated cars. I tend to get a slight momentary racing while upshifting on mine, rather than your problem.
One situation where transmissions will shudder on downshifts as you slow is temperature extremes. If it's very hot the fluid can thin. Or be too thick till warm if very cold.
Be sure to check the transmission fluid level with the vehicle in park and warmed up as per the manual instructions to see if the level is adequate. If it is, I would consider a flush of the transmission fluid and a filter change just to make sure it's not a simple fix.
If you still feel it's not right, try another Ford dealer. If the problem continues and they claim it's normal I would seek a second opinion at a quality independent transmission shop, not a Ford dealer. They have no incentive to turn you away and can/should provide a written diagnosis of the potential issues and their opinion based on experience with this popular vehicle type as to if it's "normal" or not. That will go a long way towards getting Ford's attention. Shouldn't cost too much for a quick test drive and a hook up to an analyzer.
As a last resort, one could seek out an attorney who specializes in suing car dealers. Google should help with that. Sometimes a letter from them is all it takes. Worked for my brother with a Ram that had overdrive issues early in its life. They put in a new transmission just out of warranty even paid the attorney fees!
You have maybe 3 years of warranty left on the powertrain, I would not panic and sell it as you'd take quite a hit. But I would keep trying to get it right. If in the end that doesn't happen I'd probably trade it in when it has 6 months warranty left on it. Dealers are often very flippant about test drives and may not even notice if it's subtle. A Ford dealer would take it in trade no problem, as suddenly they would decide it needed a new transmission paid for by Ford! Good luck.