How exciting! For a 2-week trip covering those four cities, I recommend 5 days in Tokyo, 4 days in Kyoto, 2 days in Osaka, and 3 days in Hiroshima (including Miyajima). Since you mentioned being overwhelmed, make sure to plan your days logically so you aren't constantly packing and unpacking. Speaking of planning logistics, if you ever decide to switch gears for a future vacation and do a driving holiday, checking out guides like this
Southern Italy and Sicily road trip can save you a lot of stress.