We already have this though. SD Express, which allows SD cards to actually protocol switch to one lane PCI-E NVME. It's been part of the spec officially since 2018, and an enhancement slightly later to add more pins to allow a second lane.
And since, underneath, it becomes a standard PCI-E NVME with standard lanes, there is no inherent speed limit from the bus itself, only from the fact that SD cards are tiny and any real controller is going to cook.
And since, underneath, it becomes a standard PCI-E NVME with standard lanes, there is no inherent speed limit from the bus itself, only from the fact that SD cards are tiny and any real controller is going to cook.