How to force boot from SD card on H96 Max X3 after Armbian is already installed to eMMC?

[chsprs]

Device Information

• SOC: S905X3, Model: H96 Max X3
• Kernel: 6.12.93-ophub, already installed to eMMC (DISK_TYPE='emmc' in /etc/ophub-release)
• MAINLINE_UBOOT='/usr/lib/u-boot/h96maxx3-u-boot.bin.sd.bin'

What I'm trying to do

Boot CoreELEC from an SD card temporarily and non-destructively (to test USB 3.0 behavior under a different kernel/PHY init — see #3601 — without touching the working Armbian eMMC install).

What I tried

• Prepared a CoreELEC Amlogic-ng SD card, with the matching sm1_s905x3_4g device tree renamed to dtb.img at the root of the FAT partition.
• Powered the box off completely, inserted the SD card, then held the reset button (toothpick method through the AV jack pinhole) while powering back on.
• Result: every time, the box boots straight back into the existing Armbian install on eMMC. The SD card is never used, no matter how long the button is held.
• Also tried a plain cold boot with the SD card inserted and no button pressed — same result.

Question

Since ophub's own u-boot is already flashed to eMMC on this device, does the AV-jack reset button still force SD-card boot on this setup, or does it only trigger Amlogic USB Burning Tool mode (i.e. it just sits waiting for a PC running the official burning tool, and silently falls through to eMMC if none responds)?

Is there a documented/safe way to force this board to boot from an SD card without serial/UART access, given the eMMC already has a valid ophub-installed system on it (e.g. a bootfromsd u-boot env var, or some other supported key/jumper sequence)? Trying to avoid bricking a device that's currently in daily use.

Any pointers appreciated — thanks!

[ophub]

Rename /boot/boot.src on the eMMC to prevent booting from the eMMC.