Ed,
I am an old music collector like you. I've got about 140,000 tracks in my music library which I began with iTunes version 1-point-something, have used and expanded constantly since then, and am now enjoying with the Music app, v. 1.2.1. My library is located on an external drive. For a few years now I've been moving to lossless files, generally downloaded from someplace in FLAC format, then converted to Apple Lossless (ALAC), though the majority of my library is still in 256kbps "matched" AAC or in MP3 at various bitrates.
There have been occasional technical hiccups, and an ever-evolving interface to adapt to, but generally everything still works pretty much as it has always worked. I don't think there's any technology-related reason why you can't continue to use the Apple music app — originally called iTunes and now just Music — as you always have.
Cheers