Hi,
If you are thinking about going to North Germany in July and are interested in history, as you expressed above, numerous places in Norther Germany have interesting museums, memorials, monuments, especially if you want to see those pertaining to Prussian-German history.
As for places in northern Germany swamped by tourists , that doesn't happen, definitely, not in the towns. Yes, some tourists are there, relatively fewer , if any. depending on the city. You will not find that in towns where I've been over and over or even once)...Lüneburg, Eutin/Holstein, Cuxhaven, Munster/Oertze, Goettingen, Kiel, Soest/Westfalen, Minden,
International tourists and Americans do not go to North Germany in droves as can be seen in Munich, the Middle Rhine area or in Bavaria,
Most likely, you might just be the only person visiting a museum, literally and figuratively, I've had that happen in North German places. "Super crowded areas" don't exist in northern Germany relatively to other areas in Germany.
Bottom line is if you want visit and explore towns not inundated by crowds, go to towns in North Germany, better still those places in eastern Germany....Halle, Jena, Meissen, Weimar, Potsdam, Naumburg an der Saale, Magdeburg, Leipzig, Neustrelitz...all well known culturally and historically.