Unfortunately, that wouldn't work for my wife's situation. Her computer is one of those MacBooks with a 256GB drive, and only one USB-C port. So the amount of data stored in her iCloud photos would easily overload her drive, and if I hooked up an external drive, the computer would run out of power before the transfer completed.
I'm not interested in buying a hub for a one-off operation.
I tried afp:// and smb:// mounting Photos libraries on large drives on other machines on the LAN, then downloading the photos to those, but that turned out to be excruciatingly slow, and very unreliable.