Omnia boot TOS from USB fails (flash is broken)

I am failing to boot a Turris Omnia (Model: RTROM01) via USB (eMMC flash seems to be broken, btrfs check showed lots of I/O errors).

I’ve updated the u-boot bootloader by downloading and serving, so start screen displays
before:“U-Boot 2015.10-rc2 (<date/time>), Build: jenkins-omnia-master-23
now: “U-Boot 2015.10-rc2 (<date/time>), Build: jenkins-omnia-test-235

I’ve created a 16GB USB 3.0 Stick following, connected the router via serial cable, and stopped the boot process after it displayed “Hit any key to stop autoboot”

When I type:

=> usb start
starting USB…
=> btrload usb 0 0x01000000 boot/zImage @
** Bad device usb 0 **

So it doesn’t find the kernel to boot, but when I start in rescuemode (=>“run rescueboot”), I can see boot/zImage in subvolume @ on the USB stick.

Any idea where the error could be ?

Meanwhile I’ve successfully tried the same with a temporary mSATA SSD, except that the boot process moans a bit about not being able to read from the degraded, internal eMMC. Otherwise it seems to boot and work as expected, but not only me would prefer booting from USB.
I’ve found a thread from 12/2018, where some else seems to have the same problem.

That leads me to the conclusion that the official documentation is misleading by narrowing the use of USB to a not further mentioned “mini PCIe to USB 2.0” adapter and U-Boot/USB-controller don’t support it (yet?).