I'm surprised not to see any mention of Clementine (https://www.clementine-player.org/) here. I switched over to it last year and it's lovely. The interface is nothing special but it's quick and efficient, and the library updates are immediate (I assume it uses libnotify/equivalents). On Windows it idles at 70MB of RAM and 0.2% CPU whilst playing FLAC files for me. Similar stats on my Linux laptop too, from what I remember. Great audio codec support and it has various features for cloud streaming services, though I don't use those.

I'd recommend trying it out, I think you'll be surprised by how nice it is to use.