flashing partitions with fastboot vs installing with SD install image is not strictly the same. When flashing with SD install the entire eMMC is reset/reflashed, including all the bootloaders and the GPT partition table. In general, I would recommend to use the SD install, to make sure that the system is in a ‘known’ state, since flashing individual partitions might put your system in an unknown state, unless if you are sure that you reflashed them all properly, including the GPT.
Now about this issue, can you please try to install the Linux SD install image? After reading the messages above, I am tempted to bet that Android doesn’t recognize your monitor. but that’s just a guess. The SD install that you see is running from a small Linux initrd, so it runs with the Linux DRM/KMS display driver, not with the Android display driver. Since you can see the installer, it at least means that the DRM/KMS driver works with your monitor.
If the monitor doesn’t work with Android, you can also try to attach with adb using the OTG USB. Give the board a few mins to boot Android, and then connect USB OTG and run ‘adb devices’ on your PC to check if the board is alive.
Of course, UART traces would definitely tell us more about what’s going on. But if you can run the installer, i believe the board isn’t defective… and that we have a s/w issue.