@Larry, as long as you follow wiki regarding supported NVMe you can expect that everything worked. Also, ambian with mainline kernel and uboot, most likely, have wider support for nvme boot
And regarding your question, I’m not seller, so I can’t say for 100%, but:
- When there is pre-order it says pre-order, like it was with SATA HAT for RasPi, so yes, you can order
- Except eMMC, which now embedded. The only difference is GHz, since it’s slightly better silicon (aka all of OP1 chips are silicon lottery winner).
- I don’t think it’s possible with rk3399, since it’s limited to 4gb by chip hardware design. Your best shot for 8gb+ is RK3588, but that around fall-2021, winter-2022