Ok, just thought to update you guys on some successes I've been having of late.
I'd upgraded my VCS to Big Sur 11.3.1 and it seems to have gone through very smooth.
In addition, I'd been tinkering with some of the devices and kexts in my build on High Sierra and I have almost achieved hardware acceleration on my
build (as an APU based unit). I've managed to enable DRM to play videos, the onboard AMD HDMI and audio processor as well as unsupported SATA.
To setup and boot into the system firsthand, I used Mojave, and the key point was that aside from obviously using an AMD-compatible Opencore build (with the new AMD kernels) I disabled HPET in BIOS and used a SSDT-GFX0.aml and the shikigva=40 and shiki-id=Mac-7BA5B2D9E42DDD94 to boot in with SMBIOS iMac19,1 because my system was a Vega 3 graphics. That's what allowed me to install Mojave, then move on to Big Sur. But BS doesn't allow Shiki boot-args so you have use a different SSDT or AGPMInject kext instead.