Wow. Scratching my head all day and your comment above brought it all to life. Using a Supermicro X11SCH with Xeon 2124G and all I had to do was enable it in the BIOS and *pow* /dev/dri/card0 and renderD128 appeared. Just before this step I did add the two extra v0.13.3 files mentioned in the top post to the "synoboot" USB, so this likely also helped.
admin@MEDIA:/$ lspci -k | grep 'Kernel driver'
Kernel driver in use: i915