I just "finished" the first portable prototype for this project: basically a scaled-up version of what I want. It's a bit bulky to really bring everywhere, but it achieves the basics: coin-cell battery operable, ESD safe, measures UV index correctly, display works as expected.

The main challenge with this first versions was the enclosure. I didn't get the tolerances right the first go-around, so it took some back-and-forth with the 3d printer to get it right. It's still not a perfect fit, as the pin headers push the lid up about 1mm above flush.

The other challenge was having to re-solder the ICs everytime I burnt the board with ESD / over-voltage. I'm still not 100% which one it was, but whatever the cause, keeping VDD connected to the TVS diode seems to have fixed it. I went through 4 ICs before I got this right. Lesson: always ensure your over-voltage protection is connected to power rails! Even during testing, when jumpers are disconnected!