There are surviving letters and archeological evidence of this decline. The takeaway is that the collapse was not sudden. It happens over a century, trade becomes more difficult as new borders form, the roads and bridges go unpatrolled and unmaintained, and the marginal cost of doing business makes the trade unprofitable.
Communication between cities and regions drop off. People leave the cities as the flow of goods dries up, adding to the spiral because there are less customers and therefore less profit, and finally the roads become impassible. People just adjusted, accepted the new situation, moved on.
Here's the website. https://fallofromepodcast.wordpress.com/
I am listening to this on my phone with CastBox.
It's hard not to draw parallels. This sign is posted 100 miles inside the U.S. border, just south of Phoenix.