[dragontamer]

PCBs are a dark art, and I focus on lower-speed (below 30 MHz) to try and avoid any issues.

I know that the faster the PCB is, the more issues you get. Above a certain frequency, inductors look like capacitors, capacitors look like inductors, and PCB-traces look like transmission lines with reflections and other such nonsense. Staying at a slower speed helps negate these issues.

Most application notes, be it from STMicro (for STM32) or Microchip, or really any other microcontroller manufacturerer, will have recommended hardware designs + their thought process fully documented.

Start there. Here's Microchip's ATMega328 hardware design notes: https://ww1.microchip.com/downloads/en/Appnotes/AN2519-AVR-M...

STM32F4: https://www.st.com/resource/en/application_note/an4488-getti...

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Study up on the "reference designs". For ATMega328p, that's Arduino Uno. For more recent AVR chips (such as AVR DD), that's "AVR DD Curiosity Nano". (See schematics here: https://www.microchip.com/en-us/development-tool/EV72Y42A)