Have a look at this OpenCore guide for a Haswell system that is similar to yours -
https://www.tonymacx86.com/threads/ga-z97x-ud5h-bk-i7-4770k-rx580-ventura-public-beta.321378/
While the EFI has been created to work with Ventura it would only require two changes in the config.plist to allow you to run Catalina on your system.
- Add IGPU empty framebuffer entry in DeviceProperties section
- View attachment 555690
- You don't need any other IGPU entries, as your RX580 is being used to drive your display(s).
- Change UEFI > APFS > MinDate & MinVersion entries as shown below.
You should already be using the iMac15,1 SMBIOS on your system.
I would confirm that you shouldn't be using the agdpmod=pikera or any similar boot argument with your RX580 dGPU.