In three days from Fukuoka to Osaka

Dear Japan Insiders

We will be traveling to Fukuoka at the End of September. We arrive on the 27th in the evening and had thought to have three days in northern Kyushu visiting Aso, maybe Kurakowa, eventually heading to Osaka, to be there on the 30th some time in the late evening.
Now we find that the Aso area is still quite closed and I hope that you have some advice on how to fill our days between Fukuoka and Osaka?! We are open to hit the road in direction of Osaka and taking some stops on the way, or spening time in Kyushu and doing the trip to Osaka all at once on the 30th.
Do you think it's still best use of our time if we stay in Kyushu or would you advice to visit some Things on the way?
Thank you in advance for your help! I appreciate all ideas and Inputs!

You could stop by Hiroshima/Miyajima on the way. Nagasaki and Okayama are other options.

Tsuwano, Hiroshima, Matsuyama

I would use a day there drop by Hiroshima. Luggage goes to the train station lockers (note that do not have many large suitcase size lockers and one of their entrance into the station, is to a shopping centre, that has lockers is not opened the whole time).
This is for Miyajima and Hiroshima.
Would spend the rest of the time in Kyushu. 3 or so days is not sufficient for Northern Kyushu. Use your extra days to see Yufuin, Nagasaki and Kagoshima.

Thank you all for your ideas! I am actively looking into all the options and names proposed I that weren't on my mind jet and try to find out, what is best for us :)
Maybe if someone knows a quiet, nature place (mountains or beach) to relax a bit this would also be great as we will be around cities a lot after meeting our friends in Osaka. So we would also like to spend some quiet time before, maybe some hiking ...

Nagasaki is a gem you really should take a look at.
https://www.youtube.com/playlist?list=PL7LcPcmoSg-gbw2yXNLK92siHyNqSNx...

If you want to see sights on the way to Osaka though, and you really want something mind blowing, go see the Akiyoshido Limestone Cave in Yamaguchi - a 300 million year old marvel with an underground river going through.
https://www.youtube.com/watch?v=DqIKC9S8TFQ

Hiroshima also has a number of good places with Miyajima being the most famous. Everyone goes to see the Itsukushima Shrine with the 'floating torii', but it really doesn't take long and the real gem is the Daishoin Temple - a place you will find unforgettable.
https://www.youtube.com/playlist?list=PL7LcPcmoSg-iCNZ6AdsKGUB-HPKNxP1...
You can also hike up to the top of Mt. Misen.
Just before Miyajima also lies Iwakuni with its historical bridge and mountaintop castle.
https://www.youtube.com/watch?v=0COOZXi66mM

And aside from Hiroshima City itself, Onomichi with its temple tour and Kosanji Temple is spectacular.
https://www.youtube.com/watch?v=97Q5EjEOXI8

And since you mention hiking, the half day hike up to Konpirasan in Kotohira, Kagawa is stunning.
https://www.youtube.com/watch?v=ley9rVIewoQ
Going to Takamatsu is also nice - it has one of Japan's finest gardens also, called Ritsurin.
https://www.youtube.com/watch?v=afx-Ju52FiY

Okayama is not far and it is famous for its castle, plus Korakuen
https://www.youtube.com/watch?v=5zAI3_N7NEM
Garden which is one of Japan's Top 3, and also the Handayama Garden is beautiful:
https://www.youtube.com/watch?v=JGQ8o7k9GbA

There is a ton of often overlooked places for you to discover.

Dear all, thank you for your ideas and proposals!
So we settle for a day visit in Hiroshima, then heading down to Takamatsu for a ramen dinner, where we also spend the night. The next day we head over to Naoshima, for a relaxed day there, also spending the night there, before heading to Koya San for two nights.
Thank you again for your support, this is the best travel forum I ever came across!!!