Cool concept, phrases with the "*ON *" pattern literalized into two stacked answers. It came relatively quickly for me when I got to CARS(ON) CITY, but that didn't hinder my enjoyment. Very neat that Tracy picked all themers that when put into their arranged pairs, read as regular crossword entries! CARS, SEAS, SURGE, CORD, HARRIS are all fine answers in their own right, so at first glance, the grid looks like nothing tricksy is going on. Well done; a touch of elegance.
The asymmetry in the center did bug me a tiny bit. I liked this concept well enough that perhaps with a few more touches, it would have been my Puzzle of the Week. I don't know if it would have been possible to have a three-word stack in the center, but that would have been a gigantic bonus. As it was, I wondered where the symmetrical entry to SURGE was. Perhaps this is asking way too much — after a quick search, all I could find that was remotely close was MASON DIXON LINE.

But of course, MAS doesn't symmetrically pair up with LINE. Rats! (Such a shame too, since Mason "The Line" Dixon is a hilarious movie name.) It would be interesting to hear from someone with a clever idea on how to discover a workable central entry. Perhaps a genius programmer, like Alex Boisvert or Bob Klahn, perhaps?
The stacked entries did force some compromises in the fill, too. Stacked entries almost always bring challenges, as we see that embodied in OLEA, ORTS, SAHEL running through theme pairs. I think some of those might have been alleviated by a little more space — counterintuitive, perhaps? In this type of situation, a 4x4 chunk like in the NW can actually be tougher than a 5x4, in that a little extra space allows for more possibilities.
I might not have even blinked at the compromises, knowing that the cool theme would likely result in a few, except that there were other bits spread around. SERE is the worst offender in my eyes, being a word very people ever use. If there had been just one of ORTS and SERE, two of the examples people tend to point out when they talk about "crosswordese," that would have been much better.
Finally, I loved several clues in here. [One pulling strings?] is great, for a piano TUNER. And SURGE(ON) GENERAL did double duty, as a nice themer, and with its clever clue. [… head doctor] made me immediately think of ENTs, which is exactly the type of misdirection I like. Beautiful stuff. Overall, such a fun solve.