Yeah I know, was quite dissapointed back when I did the GTX680 flash and was hoping for bootscreens right away :/. Only way now seems to be connecting through HDMI, for the occasional time needed that’s fine.
However I don’t think the 4K display caused the issue since only after disconnecting the secondary SSD things started working
I’m glad that my post helped a bit, credits to
@bjar for the suggestion to disconnect non-boot drives.
Btw; couldnt wait and thought; “ah f**k it” Mojave is now installed and optimising apparently