Do I need to strip old trim coating before reapplying, or can I just go over it?

You can absolutely put a fresh coat right over the old one as long as it's still bonded like you describe. I did the same thing on my truck's mirror caps after about a year and a half. Washed the trim well, wiped it down with isopropyl alcohol to strip any wax or dressing residue, let it dry, then applied the new coat straight on top. It leveled out fine and you can't tell where the old layer ends and the new one begins. The coating bonds to itself without any issue when the base layer is intact.

Where people run into a patchy mess is when the old coat is actually failing, peeling, flaking, or lifting at the edges. Then the new coat sticks to the loose bits and not the trim underneath, and it all comes off together a few months later. Yours doesn't sound like that case at all. If a spot does start peeling when you're cleaning it, you'd want to remove that section completly and start fresh there, but light fading alone isn't a problem. One tip from my redo: scuff the surface very lightly with a gray scuff pad or even a magic eraser before the alcohol wipe. Not to remove the old coat, just to give the new layer something to grab. Took an extra five minutes and the touch-up areas have held up just as long as the original application did.

As for the trim near your exhaust, that's just heat doing its thing, not a prep mistake on your part. Ceramic coatings hold up to UV really well but sustained heat from the exhaust tip area cooks them faster, same reason the paint there usually looks duller sooner. Nothing you did wrong back there, it's just the one spot on the car living a harsher life than the rest. If anything, plan on touching that section more often than the others and it'll stay consistent.

Chiming in on the exhaust question since I had the exact same thing on my wagon. Trim by the pipe looked trashed at the 8 month mark while everythign else was fine, redid it, and it faded again faster the second time too. Now I just expect to recoat that one section every few months and leave the rest alone. One thing that bit me on the touch up: tape off the surrounding paint. That stuff globs onto clearcoat if you're sloppy with the applicator and it's a pain to get off once it cures. Learned that one the hard way on a door handle.

Agree with layering over the old coat, did it on my window surrounds last summer and it went fine since nothing was peeling. One thing I'd add is go thin on the second application. I laid it on a bit heavy my first try and got some streaking that only showed up once it dried in the sun. Two light passes looked way better than one thick one. Also gave mine a full day in the garage before I drove it, just to be safe with curing.

Update: finally got around to redoing those spots. Wiped everything down with isopropyl first like the first reply said, then laid the new coat right over the old layer since it was still stuck on good. Went thin after the streaking warning and it came out clean, no patchiness at all. The exhaust area I'm just accepting as a redo-it-every-few-months thing at this point.