iOS 15 support in Big Sur

I have an iMac that cannot be upgraded past Big Sur. My iphone X is on ios 15.7. When I connect the iphone to the imac via usb, i get a message saying "a software update is required to connect to iphone". I click on install and nothing ever happens. Let it run for 24 hours, nothing. Tried reboot, made sure network is ok, all good. So I basically cannot sync my iphone anymore.

Suggestions?