Tsuwano is feasible if you get to Shin Yamaguchi early enough to get the first limited express train (Super Oki) to go there. Then it makes for a good day trip. But I donft know if I would go there if I didnft have time to drop off my luggage at my hotel in Shin Yamaguchi and catch the 8:57 Super Oki.
In Yamaguchi there are buses, but itfs a little hard to get detailed information in English. That was my experience, at least. I recently had a route to some temples all planned out using buses, but it took me considerable effort (and some Japanese reading ability) to do so with any degree of confidence. Try Google Maps, maybe. I recently had an excellent three-night stay in Shin Yamaguchi and I was looking forward to exploring Yamaguchi a little, but the weather didnft cooperate. I am not sure about taxis there and how easy it is to call them using an app. (Ifm sure you could easily get a cab at Yamaguchi Station, but how to get back from the temples Ifm not sure.)
Did you see the New York Times piece about Yamaguchi? It was oddly the #3 spot on their list of places to visit in 2024.
https://www.nytimes.com/interactive/2024/travel/places-to-travel-desti...I went to Akiyoshidai and Akiyoshido as a day trip (I liked the plateau better than the cave, actually, although the cave was interesting enough). But the buses that go there from Shin Yamaguchi are infrequent. I took the first one (and it was crowded, by the way; I got in line very early and barely got a seat), but you might not get to Shin Yamaguchi in time.
There is an excellent tourist information center in Shin Yamaguchi, if you want any input after you get there. Actually there are two. The one on the shinkansen side of the station is quite small, but if you cross over the tracks there is a bigger one. I spoke to a very nice woman at the smaller place, but for detailed bus information in English I went to the other side. Ifm sure they could help you plan a day in Yamaguchi. You are definitely going to want to find something to do if you have the better part of a day after getting to Shin Yamaguchi. I really liked it as a place to stay as a home base for a few nights (with a rail pass), and will probably stay there again. Cost performance was outstanding. But there isnft much around the station area in terms of restaurants or shops, or anything to do.
I donft know anything about Iwakuni. There are some other possible things you can do in that part of western Honshu, especially if you have a rail pass. But if you canft figure anything else out maybe you could spend more time around Fukuoka and then get to Shin Yamaguchi later in the day. I do think Yamaguchi would be lovely, though, and I donft think that getting around would prove to be all that difficult. But I do understand the need to have these details worked out in advance. I am no spring chicken myself and have learned not to leave these sorts of things up in the air, just assuming that things will work outc Ha!