I wanted to try and troubleshoot this by myself (and learn in the process) but I'm still struggling.
I've tried reading every guide and forum post that I thought would help, but obviously, I am doing something wrong.
I tried it again by changing to iMac19,1 SMBIOS and both the recommended device-ids : 3E9B0007 / 3EA50000
3E9B0007 : External display lights up, internal display switches off after the first few seconds
3EA50000 : Gives an error, cannot boot to desktop.
Basically, when an invalid framebuffer(example : 3E920000) is supplied, the internal display lights up and the external display is cloned, however, there is no graphic accceleration (7 MB issue !)
When supplied with a correct framebuffer,(example : 0000923E, 07009B3E) graphic acceleration is enabled but only the external display works, with the internal screen switching off.
Any ideas what I could do next ? I believe, I have to do the correct framebuffer bus patching but I cannot figure it out.
Would it be be possible to examine my ioreg files that I posted earlier and point me in the right direction ?