I don't remember all of the places you went, but these are some that are not on japan-guide
Chiba Prefecture:
-In Tateyama: Daifukuji's Gake no Kannon is a scenic temple built into a cliff, a castle, and a large reclining Buddha (I think said to be the largest)
-Yoro Valley is considered to be an easy scenic place to walk and see nature
Ibaraki:
- Oarai's famous torii in the ocean (Isosaki Shrine) is a bit iconic. There is also a well-known aquarium nearby.
-Tskukuba has famous science and space museums, as well as Tsukuba-san, a popular hiking place
-Ushiku Daibutsu
-Kashima Jingu is a famous "power spot" shrine
Mie:
-Akagi Castle ruins and Maruyama rice terraces are popular
-Seki-juku is one of the Tokaido's nicest post towns
- Kitabatake Garden
I think the other art islands are listed on japan-guide, but Shodoshima, Megijima, and Ogijima are the main art islands that you did not list.
If you are just interested in other islands, Kitagi Island is known for its quarry (used to get granite for famous places like Osaka Castle). It requires a reservation to tour:
https://setouchifinder.com/en/detail/515Manabe Island is known for its small historic area and as a "cat island", Shiraishi Island is popular for its nature and international villa,
If you like art, while you are in Iwate, try to visit the Miyazawa Kenji Dowa-mura Museum. It's a walkthrough art experience.