Based on the POST strings of your motherboard versus mine (https://theretroweb.com/motherboards/s/fic-486-jal-rev-c), we both have an AMI BIOS so I might be able to run a similar test for my board. You're right that syslinux 6.x should fall back if E820 doesn't work but that hasn't been my experience on my motherboard hence the reversion to 4.x, I can reliably boot with 4-16MB of RAM.