If you already have the SD card and USB, your probably better off using https://github.com/pftf/RPi3/releases/tag/v1.30 which is a open source UEFI implementation. Its far more capable of dealing with underlying os updates using your OS's preferred update method. You put the UEFI firmware on the SD, update the config.txt to point at it, and then pretend the rpi is a normal x86/uefi system and use apt/zypper/dnf/whatever for updates (including the kernel). Everything then boots/runs/updates off the USB disk.

The arm ecosystem seems to be slowly moving to some form of UEFI+grub boot for all the linux/etc distributions and away from all these platform specific boot methods.