3 week itinerary

Hi
At the end of Cctober we visit Japan for 3 weeks. We are thinking of hiring a car, 2 times, instead of using a train/bus. This is the plan ->

day from to visit estemated travel time
1 Amsterdam
2 Narita Airport
3 Tokyo
4 Tokyo
5 Tokyo - pick up car Nikko Toshogu + Nikko 2.30 hr
6 Nikko
7 Nikko Jigokudani Hot Spring Korakukan 4 hr
8 Jigokudani HSK Matsumoto Castle + Magome-juku 1.5 hr + 2 hr
9 Magome-juku/Kiso Vallei Tsumago trail
10 Mt. Horaiji hike + Hamamatsu - return car 2 + 1.5 hr
11 Hamamatsu (Suzuki msm) Kyoto (evening train)
12 Kyoto
13 Kyoto Nara
14 Kyoto or Osaka
15 Kyoto - pick up car Iya Valley 4 hr
16 Iya Valley
17 Iya Valley Hiroshima - return car 4 hr
18 Hiroshima
19 Hiroshima or Miyajima Tokyo - evening train/airplane
20 Tokyo
21 Tokyo
22 Tokyo - Narita Airport Amsterdam

Is this itinerary a nice mix of culture and nature and is it feasible? All suggestions are welcome. Also tips for car rentals, hotels etc. Is it correct that many hotels and car rentals are not yet possible? Only from March due to the financial year they maintain in Japan.
Thanks for your reactions.

For the Iya valley part I would rent the car in Takamatsu. And also return it there
It�fs a long drive from iya valley to Hiroshima and Kyoto.

I would actually reconsider and either remove Nikko or Iya valley.
If you remove Nikko you could :
- add those days to Takamatsu and explore from there some of the islands such as Teshima or Naoshima, Iya valley and the spectacular garden in Takamatsu
- go by train to Nagano and only rent the car there for your part in Nagano and Gifu Prefecture

If you remove Iya valley you could either use that time to visit Teshima or Naoshima from eg Okayama area or visit Onomichi or just add a day to the Nara / Kyoto area.
I just feel to see a valley you are adding a lot of time on transport to a remote place.

You are probably already aware but renting a car in one place and dropping it off somewhere else incurs extra costs.

I would also check if you can return cars so far away from the collection location.

Hallo Maan1975.....
You could do as LikeBike said and remove Nikko at the beginning of the trip and go to Iya valley BUT you could go with the train on the 20th and come back at night (at 7pm you would already be in Tokyo again) https://www.japan-guide.com/e/e3800.html
and you could go shopping and Tokyo the night is very beautiful, the city doesn't stop!!!!
In Niko you would only visit the essentials, which are the numerous temples, which would be a shame not to visit and would return.
I never tire of seeing the temples in Nikko and I've already done the same trip 3 times.... I don't know if it's important, but I went twice in April and once in June.
Enjoy.

As already mentioned, I suggest you rent a car locally rather than drive long distance. Drop off charges for cars can be very expensive if you don't return the car to where you got it. Renting a car can make a lot of sense in some instances, such as at Nikko to go see Okunikko (Lake Chuzenji, Kegon Falls etc) since buses are not that frequent, or if you want to go somewhere else like Ouchijuku. There are half a dozen car rental outlets around Nikko Station (but reserving online would be the prudent thing to do). At this time, it is also way too early to make hotel reservations at many places or reserve a car.