Re: Frontend media box
The Intel NUC. I know I've recommended this to you before but I still stand by it. It's designed to play media and it says it supports 4K right on the box. Those cheap android units are not very reliable, I tried one. Plus they are more limited in what you can do with them. I don't think you'll find the SPDIF output but HDMI has digital audio so I think that still satisfies your need.
Some cheaper options are the zotac boxes, you could look into them. Some are less than 100 bucks. Then you have a real PC albeit a small one. You can run windows, linux, or watever you want.