@hexdump I also have the exact constraints, I've detached eMMC from my HG680P.
I've tried your u-boot using armbian and velvet's image tester, but boot of them failed to boot, and on UART I can't see anything.
Any idea what should I try next ?
Anyway, I managed to boot custom OpenWrt and I attached the scripts that I used to generate image
run.sh
insomwrt_with_vendor_u_boot.log