Hi AC2N,
At this point in time, using the chainloader u-boot.ext, if you are using USB Flash boot it will only boot up when the USB Flash with the boot image inserted on USB OTG port and no other USB drive on any other USB Ports. If your Boot image is on SD Card you do have this limitation. I have written up my experience on Manjaro arm on Amlogic Tv box? - #748 by JFL (post #737).
On post #739 I shared how to install new Manjaro Arm on GT King Pro and how to avoid it to system freeze/kernel panic (unique to GT King/Pro).
For your info, the current linux-vim 5.11-4-1, linux-vim 5.11.7-1 and linux-vim 5.11.11-1 seems to cause system freeze/kernel panic on GT King Pro (might be specific to my device, I also have SA9HH…). For the time being linux mainline kernel works better on my device.