You could definitely shorten one of your Hiroshima days and do it in one unless you intend to study each museum exhibit in detail.

Miyajima is also doable in a morning for me - don't get caught out by missing the last ferry or you'll be screwed.

In your shoes I would say why don't you allocate a day to take the fast ferry over to Matsuyama for the day and take a dip in the famous Dogo Onsen / see Matsuyama castle (it's my town ).

You would then get to see Shikoku as part of your trip

-mrploddy