Hi and thanks Soli for this.,
This is not the right forum for a full debug of my problems but since you got to this resolution for LE and nobody else seem to know anything about this in the other forum, you might be my o flu chance.
I am trying to do the same as you but with OpenpHT Embedded, basically LE 8 but with the OpenPHT client instead of KODI one.
With your files I am finally booting into something but I still have an issue.
I am using the generic Intel 8.0.1 image that I flashed with dd. I am on 2006 mac Mini with a core 2 duo. I have reFind installed from previous attempts at getting this working, and I have to select the failsafe option of the 2 available for my USB boot. I then pick "Live", it boots, I then seem to get a graphics reset as I have multiple coloured lines appearing for a sec, but then a few seconds later I get a console with the message:
"*** Error in mount_flash: mount_common: Could not mount UUID=2303-3709"
I have tried with 2 different USB sticks, one is USB 2, the other is USB 3
Probably your OpenPHT image uses another UUID. You have to edit grub.cfg and replace boot=UUID=2303-3709 with the UUID of your image. You can probably find out what it is by looking at the /syslinux.cfg file. Or if the label of the install media is OPENPHT you could also replace it with boot=LABEL=OPENPHT
(In fact the LE install media is named LIBREELEC so you could just replace boot=UUID=2303-3709 with boot=LABEL=LIBREELEC if LE for any reason decides to change the UUID)