How to modify a sysupgrade.bin into a factory.bin?

I've looked at the image files for a number of devices that produce a factory.bin and I'm overwhelmed by the different modifications. I have the GPL'd source for my device, but it is missing *.dts files and a specific image entry. I have UART output but u-boot seems to have the bootdelay set at zero.

Rather than hacking u-boot, I think the best course of action is to see if I can modify my sysupgrade.bin into a factory.bin. Can someone point to a methodical way to approach this? Specifically, I have a mt7621 based Trendnet device. I can post the UART output (1760 code lines split into several posts) if needed.

How factory images need to be structured depends entirely on the specific hardware in question, while there are certain trends commonly used by vendor A, vendors B and C or even product Z of vendor A might do something entirely different. There is no generic answer for this.

sysupgrade images aren't really an input source for that, those are merely an OpenWrt specific 'factory' image. You do need to work on the source level to assemble the factory images as needed for your device.