Touch-up paint came out darker than the rest of the car - is this fixable?

Yeah, the fading is almost certainly what you're seeing. A 2019 that lives outside has had years of UV on that bumper, and bumpers fade faster than people expect because they're plastic and take more sun abuse down low. Fresh basecoat out of a pen is going to look dark next to that no matter how well you applied it. Give it a few months too, touch-up takes a while to fully cure and the color can shift a bit once it does. Won't close the whole gap, but it helps. On the compounding idea: it can work, but go gentle. A mild polish with a foam pad rather than an aggressive compound, because you're right to worry about the clear. Edges of bumpers and areas right above reflectors are thin to begin with, and a factory bumper probably has somewhere between one and three or four mils of clear on it. Hand polishing the surrounding area with a finishing polish would be my move. Slow, lots of lubrication, stop and check often. If after a few passes the contrast is shrinking, keep going. If it's not moving, you're not going to polish your way out of it.

As for wether a shop would bother: honestly, a bumper is the cheapest panel to repaint because it comes off the car and gets shot separately, no blending into adjacent panels needed. Around here that runs a few hundred bucks, not a full respray price. So if the polish route doesn't get you close enough, it's not a crazy expensive fix. Some shops will also just spot-blend the repaired area into the surrounding section for less. My honest take though, from doing this same dance on my own daily driver: after a couple weeks you'll stop seeing it. Everyone else already can't.

This thread is helping me more than I expected, I did almost the same thing on my own car last summer except mine was a door edge, not a bumper. Same deal though, the fresh patch looked way darker next to the old paint and I panicked a bit. What ended up working for me was just leaving it alone for a couple months, the difference shrank on its own as everything cured and weathered togather. Still slightly off if I get my face right up next to it but I honestly forget it's there now. So maybe just park it and recheck in the fall before you spend money on a shop?

For what it's worth, pen paint tends to look darker untill it's had some sun on it too. I patched a scuff on my truck bed years ago and the fresh spot looked off for a good month or two, then it settled in and I honestly forgot which side had the repair. Park it in the sun when you can and give it time before you start cutting paint off the rest of the bumper.

Same thing happened to me on my old Corolla after I patched a parking lot ding. I got so tired of staring at the dark spot that I grabbed a clay bar and went over the whole bumper with it before a light polish, and that alone evened things out more than I expected. Turns out a lot of the 'fade' is just bonded grime and oxidation sitting on top of the clear, and clay pulls that off so the panel reads a shade lighter. Worth trying before you cut anything.

wierd question maybe but does the sun fix it faster if you park it facing a certain way? Like woud the back of the car getting afternoon light even matter or does it all even out eventually anyway. My driveway only gets morning sun on one side so now I'm wondering if I've been parking my car the lazy way for years.

Update: tried the clay bar trick followed by a light polish over the whole bumper. That helped a lot, the fade difference is way less obvious now and the repair spot blends better than I expected. Still slightly darker up close though. To the guy whose truck patch took a couple months to settle in, did yours keep improving in the sun after that, or did you end up doing anything else?