15 days itinerary in Kansai Area

Hello,

I'm flying to Osaka on the 16th - 31th March. It's my second time in Japan and it will be a solo traveling with a large luggage and relaxing one as well.

Here's my rough itinerary:

Osaka Kansai airport straight to Kyoto - 4 night
Kinosaki onsen - 1 night
Okayama - 3 night (day trip to hiroshima/miyajima)
Osaka - 4 night (side trip to nara, himeji, kobe)
Koyasan - 1 night (potentially store my large luggage to a locker so that I could travel light for koyasan trip)
Osaka - 1 night
Back to Kansai Airport

My concern is the trip from kinosaki onsen to okayama as it's going to take about 3 hours and changing a few trains to get there. Do you have any suggestion? I'm happy to stop over for a night or so to check out another city along the way and also open to reshuffle the trips.

Thanks so much!

I would stay 2 nights in Hiroshima or 1 night Hiroshima and 1 night Miyajima instead of staying 3 night in Okayama and do a day trip.
Consider a day trip to Kobe as well, nice city.