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.

I've been using one of these https://smile.amazon.com/gp/product/B08JZ5FVLC for about a month on one RPI 3B+, just added two more to two RPI 4s. They've been working very well. Previously, I have used these https://smile.amazon.com/gp/product/B07NTXSJHB for a little while, but traded up to a MEAN WELL RS-15-5 for 120VAC to 5VDC switching for over a year. No problems.

I usually buy micro USB and USB-C connectors, some flexible silicone insulated wire, and solder everything.

And I always run power to the USB input.