[Help] Kindle Basic 10th Gen (KT4 / J9G29R) stuck in Fastboot loop (Fastboot 0.4 / lo

[ocirep]

Hi everyone,
I'm trying to recover a Kindle Basic 10th Generation (KT4 / J9G29R) that is stuck in a boot loop. The E-Ink screen is frozen showing the cover of the last read book, but the front light works and the device is recognized via USB.
Here is the diagnostic summary and the tests I have performed so far:
Device Details:
Model: Amazon Kindle Basic 3 (2019) / KT4 / J9G29R
Board Revision: 010427400-575-G
SoC: NXP i.MX6SLL (Rex platform)
USB Identification: VID 0x1949, PID 0x0320 (Lab126 USB download gadget)
Fastboot Version: 0.4
Firmware: Official 5.18.1 downloaded and extracted locally with KindleTool (SP01 signature / Recovery V2 classification)
Fastboot Status & Tested Commands:
fastboot devices -> G090VB06051400SJ fastboot
fastboot getvar version-bootloader -> 0.4
fastboot getvar serialno & max-download-size -> Working OK (max-download-size: 0x1c000000)
fastboot stage -> OKAY (transfers data to RAM properly)
fastboot reboot recovery & fastboot reboot-bootloader -> OKAY (accepted, but device reboots back into the Fastboot gadget mode)
Restrictions encountered:
fastboot boot boot.img -> FAILED (remote: 'locked command')
fastboot flash / erase / oem unlock / oem bootmode / oem prod / oem idme -> FAILED (remote: 'locked command')
fastboot getvar all / product / bootmode / secure -> Variable not implemented
Hardware test points are too small for me to safely bridge eMMC DAT0/CLK to ground to force NXP SDP mode (1FC9:0128). I need a purely software-based / USB recovery method.
Questions for the community:
Is there any known OEM command or signed exploit via fastboot stage for Fastboot v0.4 / Rex platform to bypass locked command?
Is there a way to force a bootmode change or trigger an update package flash directly through this Fastboot interface?
Thanks in advance for any advice or guidance!