Since the vendor provided image doesn’t enable U-boot serial output, it’s almost impossible to debug boot problems, and impossible to have an elegant way to switch kernels.
Is there any procedure to rebuild the U-boot part of the SPI flash?
How to build the NOR flash image?
I think you use ROCK 5B, you can follow this guide to build u-boot:
You can refer to how armbian build its u-boot spi flash: https://github.com/armbian/build/blob/master/config/sources/families/include/rockchip64_common.inc#L199, which will add GPT partition table.