mpv with --script-opts=ytdl_hook-ytdl_path=yt-dlp allows to watch Youtube videos with a fraction of the system load that Youtube demands in a browser.

That's a neat trick, thanks! That said--I want to use a browser, because that's how to get to the thing in the way with the least friction. So while in a pinch that definitely helps, it's not (to me) reasonable for daily use, which constrains the internet usage on that machine a lot.

Yeah, the manual way is certainly the long way around, though there are (or used to be, anyway) addons for Firefox to directly send Youtube and friends to mpv. Another snag is that you can't directly switch between video qualities, though you can put --ytdl-format into the config with the appropriate formats for your screen size.