Thank you to all who contributed to this thread, I’m happy to report that I purchased the ASRock Rack E3C246D4U and paired it with an i5 8500. Using the super secret bios menu mentioned a few pages back (and documented in my linked blog post), I’ve been able to get IPMI and the iGPU working correctly. Also worthy of note is that PCIe bifurcation works well but only in x8x4x4 mode, details in the blog post.
This was all achieved using the 2.30 bios and 1.80 bmc firmware.
https://blog.ktz.me/asrock-rack-e3c246d4u-the-perfect-media-server-motherboard/