Iphone Xs can't sync with iMac (Catalina)

Ok, I've seen similar questions asked a lot, but haven't found an answer that helps. (I've called Apple Support, too.) Using iPhone xs (iOS 13.6) & iMac with Catalina 10.15.6). Since Catalina, every time I try to sync my purchased and downloaded music from my library with my iPhone, when I connect them the "charging" icon on the iPhone flashes on/off. Even if it stays on, the iPhone doesn't show continuously (if it shows at all) in the finder window->locations. It might show for a moment or two then disappears, so the iPhone can't sync. When I go under iMac's system information, regardless of whether or not the iPhone is showing in the finder window, it shows under USB devices.

I've tried switching cables and USB ports--no difference. However, those same cables and ports are able to sync an older iPad. The iPhone charges fine into a wall or portable charger using the same cables, so I assume that the iPhone connecting port is fine.

I've used disk first aid--no change; I've even reinstalled the Mac OS at the suggestion of Support--this didn't fix the issue. As I've said, I've seen the question asked a lot here and elsewhere and not found a fix that helps. I've got to assume that it has something to do with Catalina and or iOS 13.x because it used to sync fine pre-Catalina.

Thoughts?