NewPipe is fantastic, but worth mentioning since I don't see it so far, that the one killer downside is "no support for Chromecasting/DLNA" [1] [2], which means I continue to keep the official YouTube app around for occasional use when I want a bigger screen.
It does include a built-in "play to kodi" feature, though, which fits my use-case better, personally.
But I agree that more options would be beter (Chromecast though I despise it, nymphcast).
Screen mirroring nearly completely defeats the purpose of a chromecast to me.
It has way poorer framerate, requires streaming to two places instead of one (from source to phone and then phone to cast device), requires the phone to stay awake thereby needing more power and wasting the phone display which I'll not look at, has a way worse interface for whatever app you're looking at, because what you see is a scaled up phone interface instead of one more suitable for a larger screen, ...
I essentially never use it when I can avoid it, and even when I need mirroring if I have a cable I'll use that instead.
[1]: https://github.com/TeamNewPipe/NewPipe/issues/105 [2]: https://github.com/TeamNewPipe/NewPipe/issues/668