Which then begs the question - why are Imperial internal borders depicted on maps the way, say, internal borders of other feudal and premodern states are not?
There are good maps of Medieval Western Europe between the fall of the Carolingian Empire and the Hundred Years Wars depicting internal devisions and feudal entities in its complexity. But many maps oversimplify the past, and simply color a kingdom in a certain color, ignoring the complex medieval reality.
Detailed accurate and affordable Historical Atlases started to become a thing in the 19. century.
Mass education, printing and detailed maps became available. And it was in the interest of the states to emphasize nationalism in school education.
Many early historical maps where shown in school, or used in school books, and basic historical Atlases became part of education.
In this age, our view of history was dominated by nationalistic viewpoints. In these times, historians projected their nationalism into the past. We should identify with "our" "great medieval kings/heroes" defending our "country" against enemy "nations".
Look at this map, how great "our nation" extended its borders under the glorious reign of king XYZ in the X. century. And look at the lost territories during the decline during the reign of the weak king ABC . And here is a map of our great nation today, all those beautiful colonies we have all over the map to enlighten the natives. You can see how over millenia, heros defended our beautiful homeland and extended our borders.
This is of course an entirely anachronistic way of depicting history.
While this might be a little exaggerated, maps often have a certain viewpoint, interpretation or ideology.
Often, publishers reprint improved versions of previous editions of their Historical Atlas. Some standard maps emerged in the 19. century and are still the basis for some Historical Atlases today.
For example, the "
Putzger Historischer Weltatlas " (Putzger Historical World Atlas) was first printed in 1877. Later in 1911, an extended English edition was printed by W.R. Sheperd. The publisher Cornelsen gained the rights for the German Edition after World War 2, and continues printing new editions.
This Atlas was printed 8,5 million times in multiple languages. You could find older versions on the Internet Archive.
------
Now to the Holy Roman Empire. While it is clear, that it was not as good organized compared to neighboring realms, it had internal organization and common institutions, like the "Reichtag" or the "Reichskammergericht".
While it was political weak, it wasn't backwards in other aspects. Many places in the Holy Roman Empire had a vibrant culture, with great artists and a vibrant economy with great merchants. It had great and traditional universities.
In a time, when government wasn't about serving the citizens, and instead about oppressing them, more centralized governments where better at waging war, but not better for their citizens.
The religious conflicts and the 30 years war where great tragedies. But those conflicts not only devastated the Holy Roman Empire but created massive tensions and chaos in more centralized realms. Still the Holy Roman Empire managed to find solutions to continue to exist with a population consisting of multiple confessions. Meanwhile monarchs of more centralized realms often banned religious dissidents. If you where persecuted by your local ruler in the Holy Roman Empire, you could just travel to the next neighboring territory, meanwhile the Huguenots had to immigrate over large distances. Intellectuals persecuted by reactionary ruler X could find patronage by enlightened ruler Y.
----
A more centralized Holy Roman Empire would not only change the political history of Europe, but also the entire cultural development of Germany. Far fewer baroque palaces in Thuringia and South West Germany, early nationalist writers and poets writing about entirely different topics (since in OTL, they often focused on the lack of a German Nation State and the many small states). Without the Mannheim school, the history of classical music will be entirely different, therefore the history of music will change.
A more centralized German State will change the urban and economic development. Today Germany has multiple centres, with several large cities, and many strong medium cities. Berlin doesn't dominate Germany. Many of these strong cities are either traditional trading centers or where capitals of various territories or grew during industrialization. Local rulers often strengtened and extended their own capital cities. The most extreme examples are baroque planned cities like Karlsruhe or Mannheim.
A more centralized Germany would have a stronger central city and somewhat weaker regional cities.