be advised, I plan to travel from Xiamen to Guang Xi (morning), then from Guang Xi to Hongkong (night)

Guangxi is a province, so you must have a destination city in the province. Let's say, if you wanna go to Nanning, its capital city, it is suggested to take a direct plane from XM, and you can reach in around 2 hours.

For your travel from Nanning to HK, the direct plane is suggested as well. I remember a plane departs around 18:55. Hope it is suitable for you. For night trip, it may be a little hard.