There are lots of places in Tohoku. As far as bases go:
Morioka is convenient for accessing Hiraizumi, Ichinoseki, Hanamaki, Ryusendo Cave, etc. in Iwate as well as Kakunodate, Lake Tazawa, and Nyuto Onsen in Akita.
If wanting to explore Yamagata Prefecture, Shinjo is well-placed for access to just about anywhere in the prefecture, with train lines that go west to Sakata where you can transfer to reach Tsuruoka, Yuza, or Mount Chokai or going south to Yamagata City with Yamadera and Mt. Zao, Ginzan Onsen, Tendo, etc. You can also access Naruko Gorge and Naruko Onsen easily. Hijiori Onsen and nearby scenic rice paddies are also accessible from Shinjo, as is the famous "Totoro tree".
Aizu-Wakamatsu is a good area for 4 days in Fukushima Prefecture, giving you access to the Aizu region and Mount Bandai.
Hirosaki or Aomori City are good bases in Aomori. In July you may be able to see the famous rice paddy art in Inakadate, visit Mt. Osore, explore Hirosaki City's castle and castle town, visit Lake Towada and the Oirase Falls, visit the Shirakami Sanchi World Heritage Site, etc.
If you want to visit Matsushima, Sendai is the best base. Sendai, to me, doesn't have the best nearby options, but you can access Yamadera from Sendai. Shiroishi Castle is nice. Togata Onsen, Akiu Onsen, and Sakunami Onsen are there, as well.