Cannot properly sync music from iTunes onto iPod Classic

When I try to sync music from my iMac running Catalina 10.15.7 by clicking my iPod Classic's name in Finder, selecting the Music tab, and checking "Sync music onto <my iPod>," the "Revert" and "Apply" buttons are grayed out. I thought it might be because I hadn't paid for an Apple Music subscription (which I never needed, because I only put music on my iPod from CDs I've bought, not from Apple). So I started my free, supposedly functional Apple Music subscription, but the buttons are still grayed out.

Next, I did what one user user suggested and closed the application, started it again from Finder, selected the Music tab, and IMMEDIATELY clicked "Sync," before it could be grayed out. This is not what I wanted, because now my entire music library may be sync'ed onto my iPod, while I only wanted to add one album. But the app shows a steadily declining amount of space left on the iPod, so I assume it is actually updating the library on my iPod.

Apple, what should I do to make this application function as it should?