How can I troubleshoot or fix when macOS Photos app import from a connected iPhone gets stuck on “Preparing to import from…”?

Question

How can I troubleshoot or fix when macOS Photos app import from a connected iPhone gets stuck on "Preparing to import from..."?

Devices:

Setup: iPhone is connected to the iMac with a USB-Lightning Cable

Problem:

Photos app on MacOS import from connected iPhone appears to be stuck on "Preparing to import from" and never makes progress.

Steps to reproduce:

Launch Photos
Unlock iPhone
Connect iPhone to iMac with cable
Click the name of the connected iPhone under "Devices" section of Photos
Photos shows a message: Preparing to import from "Device_Name" where Device_Name is the name of the iPhone
Wait... forever...

Additional Information:

Imports previously used to work between these devices.
The Photos Library has been repaired multiple times by opening Photos while holding Option-Command.
The iPhone successfully synchronizes and backs up via the Finder when connected.
Swapping out cables does not change the behavior.
Restarting the iMac, iPhone, or macOS Photos app does not solve the problem.
iPhone has over 40,000 items (photos, live photos, videos, etc.) and a few hundred are new items since the last import while the others are already imported.

Relevant thread:

Solution

How to get the photos to appear for selection in the macOS Photos app after only waiting 1-2 minutes

Disconnect the iPhone.
Close the macOS Photos app.
On the iPhone, open Settings.
Select Photos in the Settings App of the iPhone.
In the "Transfer to Mac or PC" section of the Photos settings, Automatic is the default. Change it to Keep Originals.
Open the macOS Photos app.
Reconnect the iPhone and select the device in the macOS Photos app.

Keep Originals Setting

Source: Photos 'preparing to import from iPhone' message....

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