You can even duplicate your config a few times changing only the smbios part and name them something like config61.plist (for MP6.1) or config191.plist (iMac19,1). You can then choose which config you would like to use at the Clover boot menu in options.
My guess is the O
My guess is you already Lilu and WG installed but are missing agdpmod=pikera. I am able to boot my 5700XT with full acceleration without Lilu and WG but choose to use them because they patch other things as well. With those however you must have the boot argument agdpmod=pikera.
I believe Apple products rely more on Intel Quicksync for rendering. If you change your SMBIOS to iMacPro1,1 then it should use the GPU but you might encounter stability issues with macOS and the Navi card.
I recently went from a GTX 970 on High Sierra to a 5700 XT on Catalina. I removed the Nvidia card and drivers. Installed the new card and updated macOS. Besides one or two different bootargs everything went smoothly.
Downloading the 10.15.6 combo update appears to have a different version of AMDRadeonX6000Framebuffer.kext than what I currently have installed with 10.15.6. Not sure if it means anything though. I didn't install the combo update to see if it changed my macOS build number.
Are we sure the OS is the same build number as the currently available version of 10.15.6? I know in the past newly released products have had different build numbers when shipped. I hadn't realized they had already released the iMacs.
This is because the current NAVI drivers from Apple are poorly written and not optimized as there aren't any desktop Macs with a NAVI gpu yet. There is an iMac coming out with a 5500 so I would expect big improvements then but not likely until Big Sur.