Alps - best direction and itinerary?

Hi all,
Brand new here, my wife and I have organised a last minute trip to Japan (and I mean last minute), we are flying today to Tokyo and have booked a week there in an Airbnb. We will fly out of Osaka on Sep 18 so have 5 weeks. We don�ft want to try and cover everything as we are not well prepared and will definitely return. We definitely want to visit the Alps region.
From our limited research the popular sites seem to be Nagano, Matsumoto, Shirakawa-go, the Alpine route, Kamikochi, Takayama and Kanazawa and the Nakasendo hike. I am guessing mid to late August will be too hot for Onsen visits.
Is there a route people recommend, it seems we could go direct to Nagano and move westwards from there (would still like to hear recommended itinerary) or we could head to Nagoya and then do Hida train? Perhaps doing Mt Fuji on the way?
I assume wither way we would train from Kanazawa to Hiroshima or Kyoto depending on time left in the trip?
As we have no idea and no bookings is it easy to find out transportation routes when on the ground? Can you simply book same day?
My apologies for the lack of specifics but as mentioned this was a last minute change of plans.
Many thanks
James

This is what I would consider:
Tokyo - Matsumoto (with side trips to Kiso Valley and Kamikochi) - Nagano - Alpen Route - Takayama - Shirakawago - Kanazawa - Kyoto/Hiroshima - Osaka

Mount Fuji could be inserted between Tokyo and Matsumoto.

I am guessing mid to late August will be too hot for Onsen visits.

Although nicer in the cooler seasons, onsen are surprisingly refreshing even in hot weather.

As we have no idea and no bookings is it easy to find out transportation routes when on the ground?

Until August 18 is one of the busiest travel seasons in Japan, and long-distance travel can be inconvenient because of the crowds. After August 18 it becomes easier, although it still tends to be busy due to the summer holidays.

Can you simply book same day?

Most trains and buses: yes. Many long-distance buses and a small number of trains require seat reservations. If there are vacant seats, the reservations can be made on the day. I recommend using the access section on japan-guide.com's sightseeing guide and the transportation section in the Planning section to do the research:
https://www.japan-guide.com/e/e623a.html
https://www.japan-guide.com/e/e627.html

I am not very heat tolerant and yes, some places in the Alps are less hot than down in Tokyo (or even worse Kyoto) but if you have the luxury to change me back to Japan I would do most of August in Hokkaido and Tohoku.

There are great festivals in Tohoku, great onsen , nice cities and rural highlights. And generally WAY LESS tourists.

You can find more detailed info on this webpage. You have some time while in Tokyo to organize or if you buy a traditional paper guidebook you can use your flight time.

Early September will still be quite hot but already a little bit more bearable. So by then you can come back south and visit Kyoto , Nara and the like. Or you could do last week of Aug in the Alps and then connect to Kyoto.

Enjoy your trip to Japan!

Thanks for the responses.
I think we will stay in Honshu for this trip although it would be a good time to go north.
Glad transport will be easier after the 18th!
We landed last night. I think after this week in Tokyo we may visit Disney Sea although the sound of the crowds is not appealing, after that like the sound of the 5 lakes area for a couple of days.
Is it possible to go from there direct to Matsumoto as you recommend?
With the side trips to Kiso and Kamikochi- are these day trips or are you recommending staying a night in each area and then returning to Matsumoto?
If we go from Nagano to the Alpine route where should we base ourselves to leave our luggage?

Thanks again for the response

Thanks for the responses.
I think we will stay in Honshu for this trip although it would be a good time to go north.
Glad transport will be easier after the 18th!
We landed last night. I think after this week in Tokyo we may visit Disney Sea although the sound of the crowds is not appealing, after that like the sound of the 5 lakes area for a couple of days.
Is it possible to go from there direct to Matsumoto as you recommend?
With the side trips to Kiso and Kamikochi- are these day trips or are you recommending staying a night in each area and then returning to Matsumoto?
If we go from Nagano to the Alpine route where should we base ourselves to leave our luggage?

Thanks again for the response

With the side trips to Kiso and Kamikochi- are these day trips or are you recommending staying a night in each area and then returning to Matsumoto?

It is up to you. If you have the time and interest and do not mind the minor luggage complications, I recommend to stay overnight in both places. If not, then they can be done in side trips.

If we go from Nagano to the Alpine route where should we base ourselves to leave our luggage?

I recommend to traverse the route and use a delivery service to send possible larger luggage from Nagano to Takayama (next-day delivery by regular delivery services) or from one to the other end of the Alpine Route (same-day delivery by the special delivery service along the route).

Uni,
Can I ask you to tell me what you think of this similar itinerary someone on another site proposed?
Fuji 5 lake to Matsumoto then to Nagano then the Alpine route (recommends sleeping in Murudo), then Kanazawa then ShirikaraGo then Takayama - side trip to Kamikochi and back then Kiso valley and onto Kyoto.
This is just slightly different to yours in that you recommend doing the side trips to Kiso and Kamikochi from Matsumoto and doing Takayama before Kanazawa?
Thanks