@Spagnoletto,
The only desktop Mac Appel released with 8th Gen Coffee Lake CPu's was the 2018 MacMini SMBIOS = MacMini 18,1, however it is a bit of a strange configuration due to its use of TB3 for dGPU's
Since there are no changes to the instruction set or CPU features between Kaby Lake and Coffee Lake (just extra cores) I would use imac 18,2 and spoof your CPU using WhatEverGreen as Kaby Lake i5 7400 using the boot flag lilucpu=9 and configure the IGPU as Headless using PlatformID 0x59180002 or 0x59120003.
For more info on using WhatEverGreens spoofing feature see :-
https://www.tonymacx86.com/threads/guide-general-framebuffer-patching-guide-hdmi-black-screen-problem.269149 https://www.tonymacx86.com/threads/guide-intel-framebuffer-patching-using-whatevergreen.256490/ English: https://github.com/acidanthera/WhateverGreen/blob/master/Manual/FAQ.IntelHD.en.md
www.tonymacx86.com
For more info on configuring WhatEverGreen for headless IGPU see:-
An iDiot's Guide To Lilu and its Plug-in's Last Update: 22nd Jan 2021 (Add note about OpenCore config.plist modifications) About this Guide Note: This guide was written when Clover was the boot loader of choice as such the config.plist examples in this guide are mostley Clover specific ... if...
www.tonymacx86.com
Cheers
Jay