SD Express is PCIe and much smaller, but a full sized SD card still isn’t small. Supposedly microSD Express exists but I’m not sure that I’ve ever seen it…
>>bpye+(OP)
Aren't the speed constraints on SD cards due to the controllerless single flash chip design rather than the connector format? Unless you also shoved an ssd controller on the SD Express card I can't see it being much of an improvement.