It would not limit you to just SSDs, the connector on the rk3588 devices generally exposes 4 pcie lanes and an adapter to make this a full pcie slot is already available.

The best part about M.2 E key is that it does not require +12V.

In addition to the ability to adapt M.2 slots into PCIe via an inexpensive adapter, there is a growing ecosystem of M.2 cards for most of the things you'd use a PCIe card for. USB controllers, SATA controllers, Ethernet controllers, FPGAs, even display adapters.