You can easily get the source packages for the likely small set of packages you actually need for your embedded devices instead of downloading some 30GB of ISOs

You can easily get the source packages for the likely small set of packages you actually need for your embedded devices instead of downloading some 30GB of ISOs of everything, most of which you don’t need. You’re basically talking about an internal mirror, and you don’t need ISOs to set up a mirror. Also, installing from source on a fleet of embedded devices makes no sense whatsoever, the software should be cross compiled on some more powerful machine. So it’s still not clear how source ISOs are helping you “flash USBs”.