Switch to a different Music library on iPhone

Having used the Music app, and before that the iTunes app, for many years to organise my music collection, I am trialling an Apple Music subscription to access the various benefits. However my main music library (along with two other libraries I created) is curated and tagged with such details as composer names in my preferred format and album covers and such, so I don’t want to sync this library to the Cloud at the risk of losing my tags. I have created a new library to use exclusively with downloaded and sync’d songs. So here’s my problem: While I am savvy in switching between libraries on my mac, I want to use this sync’d library on my iPhone, without risk of uploading my curated “main library” to the Cloud. So how do I set up the Music app on the iPhone to engage with this new library and not my “main library”?