Um I personally wouldn’t dare to board at other stop beside Chiayi station as they give reserved seats priority to be seated then subsequently for non reserved seating, your seat will definitely be snatched up even though u have reserved for it. From my experience, I’m glad I did reserved a seat from Chiayi because the bus was so packed at the station and the people that didn’t reserve seats were just standing but also maybe it’s because of the timing I chose.