I managed to sort it out :)

Apparently it was a setting in the bios which I needed to disable...
Namely : IGPU-Multi-Monitor
This is the guide I followed for my build, and that setting is mentioned under the "imac 20.2-specific settings" paragraph :
https://github.com/SchmockLord/Hackintosh-Intel-i9-10900k-AsRock-Z490-Phantom-ITX-TB3/blob/master/Docs/BIOS-settings/bios-settings.md

Thanks for your input.

Michael