micro PC + some multi-launcher? I'm sure there's some 3rd party windows/Linux app that streamlines multiple services. Then add Plex server for any custom/downloaded media. It takes setup, but that's seems to almost always be the case; either a black box or assembly required (stability never guaranteed).

Perosnally, my "set top box" is a chromecast. and then localcast/airflow for anything that doesn't have chromecast support (all my subcribed streams do, fortunately). It's google so it's not really "unaffiliated", but as long as I can mirror a screen, I technically never have to worry about contracts blocking chromecasting.