The Raspberry Pi does not have any sort of DisplayPort. It has HDMI, MIPI-DSI (which is barely documented and only officially supported for the official Raspberry Pi screen), and MIPI-DPI (which uses all the GPIO pins).
As near as I can tell, nobody makes HDMI-to-DP converters (just the other way around). There are a bare few chips that do DSI or DPI to DP conversion, but they don’t have much public documentation or sourcing, and they’re all really tiny BGA packages, so it’s far from easy.
Compared to that, PCI Express is actually fairly easy. The mining craze made simple 1-to-4-lane gen 2 PCIe switches cheap as chips, and you can just shove the signal over a USB 3.0 cable if you’re feeling lazy.