-
Create ground plane - 1 entire layer for ground. You'll just use via to connect them
-
read the data sheet it'll explain what each pin does. Most of the times single pin can act as multiple things and you need to set certain bit to use certain functions.
-
for layout fan out from MCU and then on 1 layer try to be horizontal and on another vertical.
-
when it comes to crowded board vias are both your friends and enemies so use them wisely.
-
make sure the track length, spacing between track and a via, diameter of via, etc. Are within spec for your manufacturer.
-
check if any pins are differential pair. If so again adjust your software settings. I think vBUS is a for maybe usb but check datasheet.
You may also want to check out Robert Feranec's YouTube channel he has some really good videos. Even one on full end to end keyboard design https://www.youtube.com/playlist?list=PLXvLToQzgzdftro2qK5In8p6ExsDFdpzw
