I'm happy with the outcome, but the behaviour doesn't really make any sense.
Why does my iPhone have an option to delete an album, if it will be automatically restored on the next sync? If wifi sync actually worked, the deletion would always be undone the same evening. That seems at best pointless, and at worst a bug.
There are old threads from 2013 which confirm that the behaviour used to be different - that the chosen tracks would no longer be synced to the iPhone, as though I had unticked them, which is the behaviour I ideally wanted. All I wanted was for those tracks to no longer reside on my iPhone, and that's exactly what used to happen.
What got me worried was the actual text of the delete button, which says "Delete from Library". I had been happily deleting albums without realising that "Library" probably has a specific meaning, not limited to the device I'm using. So I was concerned that my Library (i.e. including the master copy) would get pared back.
No harm done in this case, other than a massive waste of time backing up a 250Gb iTunes library. But whichever way you look at it, the actual behaviour makes no sense.