Nice work. Just so that you know, I had also been experimenting with IGPU in the past. While I couldn't find dGPU I needed to make iGPU to work at its maximum potential.
With your changes, HEVC will work, but still not at full potential. To test, download a very high bit video: Google - 400-mbps-4k-uhd-hevc-10bit and play it in IINA or VLC or both. It will stutter a lot. The frames will skip a lot. If you add "igfxfw=2 wegtree=1 igfxrpsc=1" boot args along with Casey's default config (igfxfw is most important, you can probably try omitting others after testing). igfxfw enables Apple Firmware for Intel GPU. Look for docs of WhateverGreen on GitHub for more information.
After this, the video decode will run as smooth as it should. Try playing it in VLC and IINA. It also makes it closer to real Mac. Also note that I'm using iMac20,2 on 10th Gen processor (10900K). You can also try the device-id I'm using in the post number mentioned below, it is more closer to the real iMac20,1 or iMac20,2.
To make sure Apple Firmware is loaded you can look into logs.
It needs to say "accelDidLoadGuC" and "Graphics accelerator is using scheduler: Apple Firmware", this will change from "Host Preemptive" mode:
See post #4,492 in this thread for more information and full logs.
Cheers.
