Car rental around Mount Aso/Kurokawa onsen

Good day!
I plan to have a few days in kyushu next october.
My plan is to start in kagoshima and them takachiho, mount aso, kurokawa onsen, beppu and fukuoka.

Since transportation is a bit problematic (i can take the SUNQ pass for 3 days but i still have to rely on very limited busses to and from takachiho and mount aso) i tohught about renting a car when i leave kagoshima, and then leaving it at either mount aso or kurokawa onsen and doinf the rest of the way with public transportation.
Mu main issue here is that i cant really find a car rental around mount aso or kurokwawa onsen. Can it be that there is nothing (and even if there is - i would be taking the car in kagoshima and returning it there so the one way fee might be very heavy).
Is this option really wirthwhile, or should i revery to doing it the SUNQ pass way? Or perhaps renting the car for another day until i reach beppu?

Thanks!
Ohad

i cant really find a car rental around mount aso or kurokwawa onsen.

Why would you expect to? Those places are the destinations, not the starting points. Car rental places are at places where there are enough customers to turn a profit - like airports, cities, and main rail stations.
Plus you should remember that there may be some very high drop off charges if you return the car to a different faraway place than you rented it from.

You will be able to rent a car or drop a car at beppu.

Thanks!
In this case I think we will rent the car at Kagoshima and return at Fukuoka. The total cost of the sunqpass for 2 people for 4 days will cost about as much as the car rental with the drop off fee (but without the gas😁) so this seems like the best choice...

Don't forget to include tollway fees. Since you are travelling such a long distance (in Japan!), I suggest researching the Kyushu Tollway Pass which is available for foreigners. Also note that Japan has notoriously slow roads when it comes to speed limits. I'm currently driving in Kumamoto Prefecture (to Aso tomorrow) and I have lost count how many old people pulled out in front of me today and were a little bit dangerous. This is not common in Japan - but seems to be very common in rural Kumamoto prefecture.....