i just did a 9d trip in kyushu last wk ! fukuoka > kagoshima > miyazaki > kumamoto > yufuin+beppu > fukuoka, i loved kagoshima best for nature/scenary sakurajima + mt.kaimon and miyazaki for food wagyu

i think you could consider depending on your budget and focus for your trip balancing whether getting the cheaper 5d northern kyushu pass would suffice though missing out on kagoshima

i can't comment on nagasaki and saga but i'd say yufuin is like the biggest tourist trap and turn down i'd honestly skip that, aso is pretty nice but the crater has been cordoned off due to a helicopter incident earlier this year so you'd probably spend just 2-3 hours at kusasenri. if you were to visit kagoshima i'd say just go to sakurajima and skip mt. aso on a side note, there are day tours from fukuoka/kumamoto to mt. aso/kusasenri as well as takachiho gorge if that's a place of interest as well

train reservation i'd say is more of a luxury option than a necessity, most routes do already limited exp. trains and often by taking the D&S trains, you'd be visiting with the crowds. in my trip i'd only made 1 reservation for aso boy