There is also the Membrane framework, another project by folks at Software Mansion (like Elixir WebRTC mentioned above seems to be). My company uses Membrane and it’s working well. It implements a WebRTC endpoint, among other things.
Well, we have a mature and solid implementation of WebRTC inside Flussonic. It of course contains everything: dtls, ice, rtp, rtcp, etc.
It is not really easy to opensource it even as a library, because it is highly dependent on underlying media library that handle all #video_frame{} operations.
WebRTC is a deep hole, there must be implemented lot features to increase quality of video.