I just did Kaohsiung, Hengchun (Kenting), Taitung, Hualien, Taitung, Kaohsiung by scooter ! in 8 days. Way too fast ! but hopefuly I just want to live in Taiwan so will have plenty time to do it again
But awesome, tho there is a lot of part it's only straight, full of wind, and trucks/bus or dickhead in expensive cars speeding like hell. But Hengchun Taitung it's pretty fun in the mountain, Taitung hualien I would choose the rd 11 very nice stop to do (but straight and windy), I didn't have time to explore the rd 9 in the way back, apparently some good stops to do but have to go "off the road".
And yeah I want definitively do the road 8 (Taichung/Taroko), but I did a small part, from Hualien to Taroko gorges, awesome too. And no time to explore the Taroko national park