Hi Pastrychef,
I have decided to stay with Clover for now. All is working perfectly in Clover (smooth as well).
I got OpenCore working with a different release (also the keyboard in the boot menu, I guess it has to do with conflicting kext's / efi's?)
For some reason, the IntelMausiEthernet.kext doesn't seem to work in OpenCore, my Ethernet is doing nothing. However, the same kext works perfectly when running in Clover, very weird.
There are 2 small error's in your Clover config file that I corrected. The iMac Pro 1,1 system definition has firmware 1037.0.39.151.1 but should be 1715.0.57.0.0. Otherwise, you get an annoying error message while booting with Clover (it still boots and works like a dream regardless).
The second one is inside SystemParameters -> InjectKexts. It is defined as class 'STRING = Yes' but should be 'Boolean = Yes', otherwise you also get an error while booting.
Thanks a million for all your work! Without it, I probably would be looking at 'Still Waiting for Root Device'.