Hm, ok I reworked the board a little to get the crystal closer to the xtal pins. The longer one is now reduced to 11.6mm. Doesn't look so symmetrical any more, but yeah well.
I also tried to move the bypass capacitor closer to the GND/VDD pins, but there's not much more room for improvement now without changing the layout completely.
The only option left to make the traces shorter would be to move the bypass capacitor and/or the crystal to the bottom, but I don't quite like the idea.
@nctnico I'm not so sure what you mean there. In the end, the 3V3/VDD must come from the side and go to the middle.
What I could tois to move the VDD (and GND) pin(s) to the middle on the header (e.g. GND left middle and VDD right middle). From a practical point of view, I like the current pinning better though.
Not sure if it's worth the effort just to make the VDD trace shorter.
I toy with the idea to put a debug header (10 pin SWD) on the PCB. Would be nice to have for breadboarding.