I really like the customization available with Quod Libet, but like other programs, it can really choke up on large libraries.
There is a fork of Clementine called Strawberry that has some experimental Mac builds that shows some promise. It has handled larger library sizes better than most similar programs for me, but it's still a little rough around the edges.
I usually just end up playing music from my file system using this fork of Cog. Not a lot of frills, and not library based, but plays everything, looks better than VLC, and is a pretty good day to day workhorse if you don't need a lot of bells and whistles.
If there was a library based Mac audio player that could really handle large libraries and didn't have a store that I absolutely do not want crammed into it in any way, shape or form, I'd sing it's praises night and day. I experiment with players all the time, though, and I don't think that exists at the moment.
Using less system resources is more important to me than a beautiful UI. My music player (Cog) averages around 20MB. Due to lack of updates, I was worried I wasn't going to be able to use it anymore, however, it was recently ported to Mojave.