If going to Shikoku and looking for off the beaten path, then the Iya Valley is very much that, and it is indeed beautiful, especially the deeper part of the valley. They have a pretty useful English website (a rarity in rural Japan): https://www.iyatime.com/

Having a car would be necessary to see any more than just a couple main spots, and the main road in/out of the valley didn't seem that bad, but the main road is not where people traditionally live, so you need to go up into the mountainside villages to see the old-time lifestyle. Definitely not like Tokyo! But the roads into these hidden spots are narrow, one lane, and full of crazy curves... so probably not so fun in a rental car (especially if you're not experienced with left side driving).

The valley is pretty big, so you may want to opt with a private tour instead, not just for the driving, but having a local guide would be key to figuring things out. I was there with my wife pre-Covid and the guide we had was pretty impressive and we certainly wouldn't have done nearly as much on our own.