Osaka-Yakushima cheapest route W/O JR pass

Hello!

I'm living in Osaka on a working holiday visa and my brother is coming to visit in April. I'd like to visit Yakushima while he's here. Ideally, we would like to visit Kyushu at the same time; Fukuoka, Nagasaki, Kagoshima... but since I can't use the JR pass I know it would be incredibly expensive, or slow if we take a bus.

My question is, would it be cheaper to fly directly Osaka-Yakushima and back than taking the slow route through Kyushu? I'm on a tight budget and need to figure out what is feasible.

Or, we could get the train/bus down to Yakushima through Kyushu, then fly back directly to Osaka from Yakushima.

(Or, I could take the hit of the shinkansen prices and split the cost with my brother, who can use the JR pass.)

Any advice from someone who has done the same trip without JR pass would be so so helpful! At the moment there are so many options I'm a bit overwhelmed.

You are aware that Yakushima is an island and you cannot reach it by train? I think flying is your best option here rather than trains + ferry. Both budget wise, and in order to not waste a couple days just travelling (I guess you won't want to use shinkansen)

Osaka-Yakushima flights are expensive; the cheapest option would probably be highway bus to Kagoshima plus car ferry.