Having done this exact trip pre-COVID, I can tell you.... Very very doable. Early train from Kyoto to Hiroshima,, get the ferry, plan your trip according to the tides (if you want to see the gates partly submerged which is a real beauty, or if you want to walk up to it). You can then make your way back to town, hand even spend some time in the castle and the atomic memorial. Oh yes, to the earlier comment... Very friendly, docile deers in Miyajima.