transferring music from iTunes desktop to iPod Classic

I wish to transfer all my music and playlists from iTunes-on my computer (mac os 10.14) to my iPod Classic. I have been doing this for over ten years but now something has changed and I can't figure out how to do it. (I am not trying to transfer any Apple Music songs which want to stay in the cloud, I am only trying to transfer legit bought music that lives on my computer desktop in iTunes). When I try the old method I now get a warning saying something similar to, "Do I want to remove all content from my iPod and replace content from the library?" But I've read plenty of stories online of people who've done this and lost all content that wasn't 'stored in the cloud'. (I'm old-fashioned in that I do not want to store my music in the cloud, I want it on my desktop computer that can sync with my iPod via a physical cable; I'm trying to live without WiFi). I know something is weird because when I go to sync music, I'm only given playlists choices to sync that live on the iPod and not my full bunch of playlists that live on my desktop in iTunes. Crazy! Why replace content on iPod with content from the iPod? Is Apple trying to force me to use Apple Music? Why is it so hard suddenly? Does anybody have any idea of how to do this (once) simple thing? Thanks very much for any help......