Travel Osaka to Hiroshima 3 d/2 n

Hi all
We will be traveling from Osaka to Hiroshima in late December this year, and have 3 days/2 nights to do a bit of exploring.
A rough itinerary so far:
Day 1: Osaka to Okayama/Kurashiki, visiting Hemeji castle on the way. O/nite in Okayama/Kurashiki (which is better?)
Day 2: Okayama to Matsuyama, Imbari, across the Seto Inland sea by bus (won't have time to ride it) to Onomichi (o/nite)
Day 3: Explore Onomichi in am, then train to Hiroshima.
We like sightseeing (with some temple visits), but not endless museums. (except Hiroshima of course!) We will be getting our onsen fix later in the holiday too.
Questions:
Is this doable?
What is the best mode of transport between these places?
Any ideas, suggestions or changes would be most appreciated!

I'm not sure I'm reading this right:

Day 2: Okayama to Matsuyama, Imbari, across the Seto Inland sea by bus (won't have time to ride it) to Onomichi (o/nite)

You mean you are starting in Okayama, going all the way to Matsuyama & back to Onomichi? Thats 300km on a coach

Not knowing what you want to see makes making suggestions a little difficult But assuming you mean Matsuyama how about:

Day 1: As you suggest staying in Okayama
Day 2: Early start to Onomichi. PM bus to Matsuyama
Day 3: Am explore Matsuyama. PM ferry to Hiroshima
Of course as with your original plan all of this depends on you finding places to store your luggage while you explore.

Thanks Stan.
On reading my post I can see the problem! Was thinking of going to Matsuyama by train from Okayama. Have a look around Matsuyama and then catch a bus over the islands to Onomichi.
Is this possible in a day?

I would still say no. It's two and a half hours by train & a similar time for the bus. That doesn't leave much time for looking around. I would suggest that unless you have a particular reason to visit Matsuyama you find somewhere nearer your route. There are no shortage of options.

I don't know your whole itinerary, but I hope the 3D/2N period you want to fill is after you have visited and explored Miyajima (in Hiroshima prefecture). To me, Miyajima is the #1 attraction in Hiroshima. At least 2 days should be allocated for it.
https://www.japan-guide.com/e/e3401.html

Hi Stan
Thanks for your suggestions. On looking at the map, perhaps we should skip Matsuyama. I'd still like to do Himeji, Okayama & Kurashiki and Onomichi. What else would you suggest along the route to visit? And where to stop for the second night?
Thanks.

Hi Nanjak Ore
We will be visiting Hiroshima city after our 3 day/ 2 night trip from Osaka. We have 3 nights booked in Hiroshima with the option of a 4th night. We may take up your suggestion and spend the extra night in Miyajima if you think it's worthwhile. Any suggestions on places to visit, either on the island or travelling from Osaka would be greatly appreciated!
It is difficult trying to work these things out from afar, not being familiar with distances etc.
Thanks

With The caveat that I have not visited at that time of year.

I would stay at Okayama and visit:
Kurashiki for the Ohara art museum & canal district
Bitchu Takahashi for the temples & Bitchu Matsuyama Castle
Naoshima island for the art galleries

Move to Onomichi & explore
Tonomoura & Takehara (If the railway is running by then)

Finally to Hiroshima for the wonderful city, Miyajima The bridge at Iwakuni etc etc

These are the places that come to mind immediately but as I say I havent been in December. I'm sure others can suggest other places. I do quite a few muesums so I have been to Kure for the Battleship Yamato museum.

Thanks Stan for your suggestions. That sounds like the way to go.
One more question - would you get a JR pass for getting around? After Hiroshima we will be heading to Nagano, so there is a fair bit of rail travel involved.
Thanks!