If you want an incredibly easy setup, the old Chromecast and the program Airparrot have been working flawlessly for me. I know you say you don’t want mainstream, which I take to mean you don’t want ads, smart home listening devices, etc. The black chromecast (without the remote) is exacly what you want. You can mirror anything from a PC or phone over your wifi. The slight delay makes it unsuitable for gaming, but you don’t need that anyway.