Have the same problem from time to time - seems like when an iOS device is trying to sync/backup over wi-fi for me and the process gets interrupted that MacOS continues to tie up resources in an attempt to continue to sync/backup. The finder stays stuck trying to sync the iOS device. I can't even shutdown the system normally because this process runs constantly and is stuck.
The one workaround I have found is to go into Activity Monitor, find 'AMPDevicesAgent', select it and click on the information button, and quit the process. Then I can eject the iOS devices on finder.
A second workaround could be to uncheck the 'automatically sync over wifi' box but then my iPhone and iPad wouldn't sync to the Mac for my music and such as I don't subscribe to Apple Music.
As mentioned, it is only a workaround. I have to restart the Mac in order for it to 'find' the iOS devices again and sync them.