Radeon Pro 580 and Blender 2.8 I know about the whole "stop using Blender on Mac thing" but still -I do. I use a iMac 27" 2017 with Mojave and have a Radeon Pro 580 8GB GPU -and it works really fine with 2.79b! But now, when I try the 2.8 beta, no GPU is found. As far as I can tell the Radeon Pro 580 has OpenGL 4.6.

Will my GPU work in later builds?
Its not the GPU's Fault. More like Apple's support for OpenGL/CL.

the RX 580s run very well under Windows and Linux. You can either use dualboot or a Virtualmachine using HW Passtthru to use blender thru for example Linux with direct Hardware access to the gpu.

Regarding the second Question: You should mostly never render to Video directly. You may try a newer 2.8 Build or a different codec.