I also agree you can make Kawaguchiko a day trip. You can use that time to see other impressive sites. A day trip to Nikko would be wonderful, or go see Kamakura/Enoshima for a day.
For the 18-23 you give yourself a good chunk of time to see the area. Since you are already seeing Tokyo, you can largely skip Osaka during the day. But at night the city comes alive and has some great places to see, such as Dotonbori, plus the night views from the Umeda Sky Bldg and Abeno Harukas Bldg are stunning:
https://www.youtube.com/watch?v=COJ_MeXbeow
https://www.youtube.com/watch?v=xb1wuyAGgyQ
https://www.youtube.com/watch?v=-AyUZmGbW8g
You can use the time to see other nearby places like HImeji, Nara, or even go visit Koyansan for a night. It'd be a unique experience you will never forget.
Hiroshima is a nice and different break as well - although again 2 days is plenty. You could use some of that time and visit other nearby areas like Iwakuni, Onomichi, Kurashiki or Okayama.
For Beppu, there are things to do there aside from the hot springs, the Hells Tour and Takasakiyama Monkey Park/Aquarium are two examples, but you could also visit another close hot spring place, Yufuinif you wanted, or go visit the Kinosaki Peninsula, which is off the beaten path and quite nice. If you are interested in hot springs, then one of Kyushu's finest is Kurokawa which is a tranquil forested area known for its outdoor baths. It entails riding a bus and paying for it, but it will be one of the finest hot spring experiences you will ever have.