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...
---------------
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)
I made a sensor board the other day (I'm just printing the case for it now), and it was very enjoyable, and even came assembled for $1.7 per board:
https://gitlab.com/stavros/sensor-board
Feel free to email me if you have any questions or just want to chat.
Also, I don't think I've ever wanted something in my life more than this badge thing.
That's fair.
Lets put it this way: if your circuit works on a breadboard, you don't need to know anything about PCB design. The PCB will pretty much always be better than the breadboard.
Things get troublesome as you enter mixed-signal (analog + digital), or high-frequency.