Thank you for your guide. I have updated my Clover/Mojave with a clean install OpenCore/Big Sur 11.3.
My system is a Z77MX-D3H-TH with a RX580, iGPU headless, SMBIOS imac15,1, big sur 11,3, OpenCore 0.6.9.
Everything is working very smooth and I'm happy. Thunderbolt Belkin dock and my DAS are working OOTB.
Three questions though :
Internal Audio:
My chipset is different (VIA VT2021) - I managed to get it working with alcid=9 in bootarg (7 works aswell but it's messy). I think I screwed up the part in DeviceProperties. Can somebody help me fix this, so I can make it more permanent ? - See attachement. My chipset info :
gfxutil -f HDEF
00:1b.0 8086:1e20 /PCI0@0/HDEF@1B = PciRoot(0x0)/Pci(0x1B,0x0)
VIA VT2020_2021 0x100100, layout 5, 7, 9, 13
Ethernet:
I tried adding the kext in the folder but it didn't work. I think it's not enough? What else should I add in my config.plist? - very noob question i'm sorry

i used to use Clover Configurator for that kind of stuff
Boot Menu :
I use a 4K monitor. The boot menu is very small (a 1080 frame in the middle of the screen). I would like to make it bigger and full screen, but can't find the settings.
Thanks for your help.