Osaka day trips book Shinkansen?

I’ll be travelling between cities from Dec 29 – 31 from Fukuoka to Hiroshima then Osaka. Reading the subs here looks like it’s best to book ahead of time Shinkansen tickets for our group of 5. From Jan 1-5 the plan is to do day trips to Himeji, Kobe, Awaji island on various days, and Google maps says to use Tolaido Sanyo line which is a Shinkansen.

Do I really need to book those as well? I have been hoping i can leave some flexibility and not have to fix the dates of the day trips and pre-book the tickets.