Kyushu and Yaeyama Islands Itinerary Check

Me and my friend are heading over to Japan to watch the final stages of the Rugby World Cup. But before that we have approximately 10-11 days starting from October 14 to spend our time in the country.

On our last trip to Japan, we really fell in love with Kyushu; especially the wonderfully enchanting Yakushima. We've decided to return and explore the places we weren't able to the last time around due to scheduling constraints. We are also keen to explore some of the Yaeyama Islands (Iriomote in particular!)

We will arrive in Kagoshima Airport (via HK) on the afternoon of Oct 14 and will be bookending the itinerary with a trip to the Yaeyama Islands.

Being avid hikers, we are looking forward to escaping the cities and doing as much hiking (and some post hike onsen time) as we can on Kyushu and do some stream trekking in tropical Iriomote.

We shall be renting a car in Kyushu (buying the Kyushu express pass), electric scooters on Ishigaki, and a car/scooter on Iriomote.

Here is what we have so far in our itinerary -

Oct 14 - Arrive at Kagoshima Airport/Rent Car/Drive to hotel in Kirishima region
Oct 15/16 Hiking in Kirishima Kinkowan National Park (Stay nearby in Kirishima Onsen - perhaps a night at Ryokojin Sanso )
Oct 16 Drive to Mt Aso. region - stay at Aso backpackers (seems like a long-ish drive to get there)
Oct 17 Hiking around Kuju Mountains. Stay at Onsen in Minamioguni (possibly Ryokan Hozantei)
Oct 18 Drive towards Takachiho Gorge/ Detour to check out Nabegataki Falls/Hells of Beppu/Takasakiyama Natural Zoological Garden/Overnight at Gokase Campsite
Oct 19 A long day of driving to check out the Miyazaki Prefecture coast line
Umagase/Sun Messe Nichinan/Udo Shrine/Ishinami Beach/Kojima Monkey Island/Cape Toi
Overnight at hostel in Nichinan

Oct 20 Drive from Nichinan to return car at Miyazaki Airport early on Oct 20
08h45 flight to Naha (OKA)
30 mins connection at Naha Airport to catch flight to Ishigaki Airport ,(both the flights are with Solaseed -> I assume this is sufficient connection time?)
Arrive in Ishigaki and rent electric scooter to travel around island
Late at night, try to find a spot in the island that televises Man United v Liverpool at 00h30 in the morning :D

Oct 21 Either more island touring on scooter OR ferry to Taketomi Island and spend the afternoon there
Catch last ferry to Ohara, Iriomote
Oct 22/23/24 Exploring Iriomote
Oct 25 Morning Iriomote to Ishigaki Ferry/Return flight directly to Haneda at noon from Ishigaki Airport
Oct 26/27Yokohama for the RWC Semi Finals
Oct 28 Free day for a day trip
Oct 29 Return international flight at 00h30 from Haneda Airport

The only modification I'm considering is leaving to Ishigaki one day (on Oct 19) earlier but that would involve catching a 10am flight from Kagoshima airport (Which would be a 2.5 hour from where we would be at the Gokase campsite)
That would mean we wouldn't be able to do any of the things on the 19th of course but does give us another extra day in Ishigaki.

Does this all sound feasible? Have I overlooked some great places?

I'm open to any ideas or critiques of course :)Thank you!

You certainly see a number of good places in the region. Depending on your travel pace, interest, and available time, there certainly are some other good places in the area to see.
Northwest of Kirishima Kinkowan is Sogi Falls, and if driving it is not too hard to get there.
https://bit.ly/2md92OR
There is also the Kyusendo Limestone Cave, one of the best on the island.
https://bit.ly/2mhzoiT
https://kyusendo.jp/
The Mt Aso area has a lot as well. It's doubtful you can walk up to the crater, but there is always more. You already mentioned Nabagataki, and if driving and the weather is good, the Daikanbo Lookout is spectacular.
https://goo.gl/RwpnU8

Ishigaki also has the Ishigaki Limestone Cave which is very impressive.
For more hiking in the Aso and Kuju area, you could try going to see Kikuchi Gorge, as well as the Tadewara Wetlands, plus more scenery at the Kokonoe Yume Otsurihashi bridge. For Beppu, you could do some exploring around Mt Tsurumi.
https://goo.gl/rmiMZH

Though there is still a lot more to see and do in the region.