Thanks, so if I set a CC CV buck converter like this to 12.6V and 7.5A and connected the input to the USB PD module, it would only work for 20V/5A and 15V/3A since it's a step-down.

I'll probably still create another compatibility charging circuit for 5-20V PD at 2A using this buck/boost converter.

500 watts is only 4.17 AMPs, you can get a 12 AMP version of that same buck converter which would give you plenty of headroom.

You could also daisy chain multiple diodes together in series. Cut the leads short, solder like 3 - 4 of them in series and then connect and read the voltage on your meter before adding the Amplifier. Keep adding until you get to a safe voltage.

When you get it low enough, shrink tube the diode chain and it won’t look bad.