Finally tried to hook up an AM4 chipset to a raspberry pi (someone suggested this on the pi-pci repository a few years ago), and it turns out it mostly just works.
only SATA and the PCIe switch properly work so far though, the ethernet driver fails to initialize, and USB doesn't detect any devices (on the ports where it should)
PCIe card housing AMD chipset unlocks more connectivity on any motherboard, including Intel models — or you can give any B650 motherboard the top-tier connectivity of X670