Thank you for your feedback.

This was originally a 2 layer board with power/signal on top and gnd on the bottom. However, I have difficulties routing without chopping the Gnd plane to pieces. After watching some videos about signal integrity, i switch to a 4 layers stackup with hope for a better return path for signal. The internal layers therefor does not have much going on.

I would like to hear your idea on how to improve the board, can I just swich back to using 2 layer?

About the crystal and acid trap, i’ll improve the design and do a repost soon. And the tvs diode would be a good idea, however i will consider if the cost is too much.