Could the boot process be?: ZSBL (Zero-Stage bootloader) -> SBI -> Kernel

Yes. It works perfectly fine on QEMU this way, but only because QEMU already loads the kernel into RAM. OpenSBI ("fw_jump") in this case just does some firmware setup and simply jumps to the kernel.

On normal devices, your kernel is likely located in a file system. OpenSBI has no drivers to read that file system and cannot load the kernel. Here you need something like U-Boot that comes with the needed drivers.

I think these slides (from page 14) are a good resource.