So, no diagnostics & recovery via USB cable (without device disassembly)...
That makes me wander how smart are my future plans (after @katadelos publishes new jailbreak

) to edit that ("YJReader-impl.jar") JAR file that enables parsing of CONTENT_FONT_RAMP file for changing font sizes for KFX books

If that code is even still there on newer firmware.
My PW3 is still on fw 5.9.7, I've done the edit at the time that was the latest firmware.