I think you will be fine in July in Poland. I've traveled in Poland in July a few times and never had an issues with weather or tourists crowds, it's still a relatively undiscovered European destination. I think anywhere in Poland will be less crowded than Budapest, Prague, or Vienna. As for summer rains in Poland they are usually quick and actually refreshing showers that freshens up the ambiance.
If I could only pick one country to learn about WWII history as a whole, I'd choose Poland. There are a plethora of sights that cover almost every facet of the war. Here are some of the potential WWII sights you can experience:
Holocaust history (Treblinka, Auschwitz, Majdanek, Schindler’s Factory)
Museum Of The Second World War (single best WWII museum, tells the story of the global war)
Wolf’s Lair, Hitler’s Wartime HQ (site of failed July 1944 plot to assassinate Hitler)
Westerplatte battlefield and museum (The battle that started World War II )
Warsaw Uprising Museum
Project Riese/Książ Castle (massive secret underground Nazi base)
Various V-2 missile sites
Stalag Luft III POW Camp (of The Great Escape fame)
Outside of a difficult 20th century, Poland has a rich and varied history stretching back over 1000 years, of medieval villages, evocative castles, royal renaissance towns, fascinating culture, great food, bucolic landscapes, and primeval forests and mountains. I'd be sure to add some non WWII sights like Tatra Mountains National Park, Malbork Castle (largest in the world), and Zalipie Village for example. Just so you don't walk away thinking the beautiful country of Poland is only about doom and gloom.
I'll let you in on a secret... there is one region on the border of Poland and Czechia in the Sudeten Mountains that combines almost everything you are looking for in a relatively compact place, it's called the Kłodzko Valley. It's located halfway between Prague and Wroclaw. The landscape is dotted with forgotten medieval villages, baroque spa towns, grand old castles, Prussian fortresses, and secret Nazi installations. This region has been at the center of a constant tug-of-war since the middle ages, between the Germans, the Poles, the Czechs, and the Austrians. The best part it is totally undiscovered by foreign tourists. I was the only non Pole in the whole region I think haha.
You can base yourselves in regional capital, the medieval town of Kłodzko, which is nicknamed as the “Little Prague”, definitely the baroque architecture of the town reflected that. You can also homebase in the spa town of Polanica-Zdrój (like I did) and explore from there. As for hiking/nature in the Kłodzko Valley you have the Stołowe Mountains National Park, a primeval forest deep in the Sudeten Mountain Range, dotted with some very unique rock formations (they actually filmed one of the Narnia movies there). There are some pretty good hiking spots here, between the rock formations themselves.
Here's a great blog post about the Kłodzko Valley https://www.travelsewhere.net/klodzko-valley-poland/
Here is my trip report + photos from the Kłodzko Valley and surrounding area (I went in September) https://community.ricksteves.com/travel-forum/general-europe/vicarious-travel-poland-2017-wroclaw-silesia