So there's mini PCI-E and there's mini PCI-E(SATA). The SATA one is just directly connected to a SATA controller, when you look for storage devices for the mini PCI-E format that's all you find thanks to it's prolific use in laptops.
The other mini PCI-E is just a 1x PCI-E channel... or a USB connection, when you look for items to connect to mini PCI-E you find Wifi and 3g/4g cards and something like this: https://www.amazon.com/dp/B07BJ45JXD. Now one day in my searches I did find some Intel Flash drives that were used for ReadyBoost duties on some Windows laptops, but they lack a flash controller and you need a special Linux kernel module to even use it which breaks my want to boot from the mini PCI-E device.
The other mini PCI-E is just a 1x PCI-E channel... or a USB connection, when you look for items to connect to mini PCI-E you find Wifi and 3g/4g cards and something like this: https://www.amazon.com/dp/B07BJ45JXD. Now one day in my searches I did find some Intel Flash drives that were used for ReadyBoost duties on some Windows laptops, but they lack a flash controller and you need a special Linux kernel module to even use it which breaks my want to boot from the mini PCI-E device.
Since the industry went the m.2 route the mini PCI-E slot is dead and dying. You can even get m.2 adapters for Wifi(https://www.newegg.com/fenvi-fv-ax200h-m-2/p/0XM-00JK-00063) or FPGA(https://numato.com/product/aller-artix-7-m-2-fpga-module) because it's more universal than a standard PCI-E slot now.
This ramble is to nominate mini PCI-E as a cursed connector.