Thanks all for the input, here is the final version :

Arrival : 20 Apr Departure : 03 May

Golden week: 29 Apr - 06 May

20-23 Apr : Osaka (3)
23-25: Hiroshima (2)
25-26: Miyajima (1) (Ryokan)
26-30: Okayama (4)
● Kurashiki as a day trip
● Takahashi as a day trip
30-02: Kobe (2)
● Himeji as a day trip
02-03 May : Osaka (1)

Hotel wise I already booked the Hilton in Hiroshima and a Ryokan in Miyajima (but without dinner to have the freedom to roam around for photos at sunset). For the other hotels I plan to use Dormy Inn as much as I can as I like this chain.
Train wise a JR pass makes no sense but it might be that two 5 days Kansai Hiroshima passes work.They are 170000 yen for 5 days.
But based on this I can not see if they cover Okayama-Takahashi
https://www.westjr.co.jp/global/en/t...sai_hiroshima/