Need a minimal circuit to obtain 5V from discrete 5V to 20V (5V, 9V 12V, 15V or 20V) input
The simplest way to do it is with a buck-mode switching regulator set for 5 V output. At input voltages of 9V, 12V, 15V or 20V it will step the voltage down to 5 V. At 5 V it will simply pass the input voltage through, providing it can do 100% PWM and has a PMOS pass transistor.
In 'pass through' mode current flows continuously through the pass transistor and the inductor, so for low voltage drop they need to have low resistance. This can be assured by using a regulator circuit which has much greater maximum current output than you need. One example of a suitable module is the XL4015, which only drops ~0.05 V with 5 V in and a 300 mA load.