That's great. Do you know where I can find instructions on how to flash Tow-Boot to the SPI and how to configure it?
Add.: never mind, I found that Tow-Boot has excellent documentation for the whole procedure. Thanks for the heads-up on Tow-Boot.
I'm sure, since my RockPro64 has no eMMC (I didn't order one, and I just checked that my eMMC slot is empty).
I have Tow-Boot installed on my SPI, which when booted jumps to the NVMe device.
My initrd has modules "nvme pcie_rockchip_host phy_rockchip_pcie" which are needed for Linux to detect and then mount the rootfs.