YouTube does offer 720p with pre-muxed audio though, so if 720p is OK then NewPipe+LocalCast should be at least decent. For 1080p video, YouTube separately provides the video and audio streams, so the downloader can't download videos with their audio in full 1080p.

If video quality is a major concern for you, this probably isn't a great option, as NewPipe can only download in the qualities YouTube directly offers videos. You can also set this to always do one specific action, so going from a YouTube video to sharing to downloading can be a pretty smooth process if you want to set it up that way. the official YouTube app) and share videos to NewPipe which will trigger a dialogue box asking whether you want to watch full screen, in a pop up, download, etc. You can use the Share mechanism from elsewhere (e.g. So you could use NewPipe to download videos to your device, then LocalCast to stream them to your Chromecast while playing the audio locally. you literally cannot log in via NewPipe).

NewPipe is basically a YouTube scraper which can play or download YouTube videos without anything Google-related going on (e.g. It's a longer process, but you could use an app like NewPipe, available on F-Droid but barred from the Google Play Store because it doesn't play by their rules for using YouTube in apps.

Download localcast for chrome