USB-C connectors can carry Thunderbolt which is essentially external PCI-Express, so the adapter might have to be very minimal if you’re fine with it only being compatible with Thunderbolt-capable devices.
A Thunderbolt flash drive in the form factor of a USB-C YubiKey 5C Nano [0] would be the ultimate solution for me. I trust that some day we'll get there if it's not superseded by some other standard by then.