I'm looking into "downgrading" to a 2017 radio, myself.
I know for a fact that the antenna on a 2018 "S" is different than a 2018 with satellite radio. I assume that all 2017 antennas are the same as the non-S 2018's.
So not only would you need the 2017 radio, screen, and trim, you'd also need the antenna and associated cabling in the roof. Plus all the radio harnesses from the 2017.