Optimal route from Fukuoka to Miyazaki

Hi. I am planning my June trip to Japan and right now have decided to fly to Fuokuka, stay in Kyushu for 6-7 days and then fly to Osaka for the rest of the trip (6-7 days).

In Kyushu, we want to hit the following spots, so would like to have a recommendation on the ideal sightseeing/day-trip route and which city we should stay in along the way. Planning to drive as we are a family of 4 and I have driven in Kyushu before which wasn't too bad.

Current thinking:
- 2 days: Fuokuka/Hakata area
- 1-2 days: Sasebo/Huis Ten Bosch area (drop by Shirato Waterfall enroute)
- 1 night of Onsen (somewhere)
- Drop by Takachiho Gorge enroute
- 2-3 days: in Miyazaki/Aoshima/Sun Messe Nixhinan areas
- Fly out to Osaka (in the afternoon)

Question 1: For HTB/Sasebo from Fukuoka, is Sasebo a day trip kind of thing or should I stay there overnight? If so, should I base there to HTB, or should I also stay at one of the 5 official hotels in HTB, or if anyone have recommendations for HTB-area hotels (if we don't base in Sasebo?)

Question 2: We want to do 1 night of onsen - is there a recommended spot between HTB and Miyazaki? We have been to Beppu/Kuorkawa onsen areas before so prefer to try something new, but don't mind repeating it if that's still the best enroute spot.

Question 3: Between HTB and Miyazaki, it's quite a drive and ideally I want to break it down into a couple of destinations 1-2 hours from each other and stay overnight at one of them. We have been to Kumamoto / Nagasaki so wondering, beside there, is there another good spot for a stay (ideally combined with onsen per above)?

Question 4: If we want to do a day trip to Takachiho, but since we have a family of 4 (2 kids), I guess it's not possible to do the boat ride right? If so, is it still worth it to go and what's the route on-foot? How many hours to budget?

Question 5: Besides Takachiho, are there other areas that we should check out enroute to Miyazaki?

Question 6: Since we want to check out Aoshima/Miyazaki, was thinking that we can fly out of Miyzaki airport, unless anyone have a better recommendation?

Thank you in advance for all your help!

That's a lot for 7 days.

I would try for an onsen between HTB and Takachiko, so you ain't wasn't time backtracking for day trips.

Amagase comes to mind. Obsen town based around a river.

Or Hita if you like a little bit of history. Less onsen town but has bath options.

Maybe Yanagawa for boat rides in old canals. Some onsens dotted throughout the town also.

I did an overnight in Takachiho, where I stayed at a Ryokan near the Amano Iwato cave shrine. One of the benefits of that particular Ryokan was a complimentary bus to the nightly Kagi Dances at the Takachiho Shrine. This may work as your Onsen.

I only stayed two nights in Miyazaki, so I understand staying an extra night there. It was the one place that I felt I didnft spend enough time.

Since you have kids, you might want to include Beppu, and go to the gHells of Beppuh. There is also a monkey park and an aquarium.

https://www.japan-guide.com/e/e4700.html

One other option that you might consider is taking an overnight ferry back to the Kansai Area. There are two, but I think the Beppu to Osaka may work for you.

https://www.japan-guide.com/e/e2355.html

I also did HTB on that trip. I did get an international drivers license for this trip, but I ended up using public transportation. I started in Fukuoka and did some day trips (Nagasaki, HTB, Arita). I then took a Shinkansen down to Kagoshima and over to Miyazaki. I took a Limited Express Train up to Nobeoka, where I caught a bus to Takachiho. I then took a return bus back to Nobeoka, and a train to Beppu.

I hope this gives you some ideas.

Good Luck, in planning your trip.

Question 1: If there are no evening events such as fireworks, I think a day trip is fine.
I stayed at Forest Villa.
Kids might enjoy a Henna hotel full of robots.
Sasebo only knows what Sasebo Burger is famous for.
Question 2: I've never been there, but Takeo Onsen, Ureshino Onsen, Yamaga Onsen, etc. seem to be good places to stop along the way.
https://www.japan-guide.com/e/e8385.html
https://www.japan-guide.com/e/e8380.html
https://yamaga-tanbou.jp.e.zh.hp.transer.com/
Question 4: I think the Amaterasu Railway is a kid-friendly attraction.
https://amaterasu-railway.jp/
https://takachiho-kanko.info/en/sightseeing/
I went to Takachiho for a day trip, but if Kagura is held at night, I think it would be fun to stay overnight.
https://takachiho-kanko.info/en/boat_kagura/#lnk_kagura
Question 5: Kusasenri in Aso is recommended for a drive if the weather is good.
https://www.japan-guide.com/e/e4552.html
Udo Shrine was also good, although it was earlier than Miyazaki.
https://www.japan-guide.com/e/e8032.html
I think children will enjoy Miyazaki City Phoenix Natural Zoo as well.
http://www.miyazaki-city-zoo.jp/univ/en/park/index.html
There is also a famous resort Seagaia nearby.
https://seagaia.co.jp/index_en.html

For the onsen, Kurokawa in one of the best on Kyushu. But if you want to try something new, there is also Unzen, which is an undersung gem of a place, or several in Saga which were already mentioned There are also some places on the west side of Aso as well, like Dondokoyu, Suzume no yu, et. al. It also makes a great place for an overnight - there are many good sights in the area, like the Daikanbo Lookout, Kikuchi Gorge, Yamabuki Springhead, Nabegataki Falls, and Kamishikimi Kumanoza Shrine.
If you can put in a full day at Sasebo, then so much the better. If the weather is good (June has the rainy season, so lots of rain), then look into the 99 Island cruise as well as go to some of the really nice lookout points to see the bay (Tenkaiho, Udogoe, Ishidake, Yumiharidake, Funakoshi) - if you have a car, that is ideal. As long as you don't arrive too late in the day, you can certainly ride the boats at Takachiho.
And you can easily fly between Miyazaki and Itami in Osaka - that would be the most convenient by far. There are 5 flights a day on JAL, and you can use their cheap Explorer Fare for foreign tourists - just 11,000 yen per person.

Unless you absolutely must go to Osaka, I'd consider spending the 14 days in Kyushu, it's amazing and with 4-5 weeks there, we didn't see all we wanted.
With 2 weeks you could see the likes of Mount Aso which is spectacular, Kagoshima, Yakushima etc.