Why not just replace the whole head unit instead of tacking on another in front of the Sync? They sell units that come with can bus adapters and such so you keep your steering wheel buttons, dims with headlights, etc...
There are single DIN and dual DIN depending on the one you're removing. Most are 7" screens because that's the width of a normal DIN unit. Worst case you have to fabricate a small trim panel to cover any gaps, vs custom wiring and custom mount.
Unfortunately the 2015+ are far more complicated than that or I would.
I looked into getting the 8" trim and switching to the new HVAC and putting a new head unit in there.
Unfortunately there are zero vehicle adaption harnesses available for the 15+ models. If you are aware of one please let me know.
Now that the HVAC module is tied into the system complicates things.
A company has made a work around for the F150s and uses a 12'' Tesla style screen that also incorporates the HVAC controls.
http://www.linkswellinc.com/products/t-style-radio/ts-fdpu12-1rr-8-t-style-ford-f-150-12-1-system/
Unfortunately they have been unsuccessful in getting this to work with Expeditions due to not being able to control the rear HVAC.
I don't have the time needed nor am I by any means savvy enough to go hacking up the factory harness and getting something to work. Wish I was but electronics is far from my wheelhouse.
A simple piggyback system why not as seamless as I would like will yield me the results I am looking for with at this point only loosing steering wheel controls. Though dependent on what I can find with the wiring may be able to pull it off with just tapping into the harness and not hacking it to pieces.
I am looking for something easily reversible if I run into an unexpected snag. So for me that means no cutting of the factory harness.