Thanks for the info. From watching various YouTube videos, there are plenty of things I would like to see and do in Hong Kong. I would prefer to spend an extra day or 2 and feel I've not rushed around to see things.
For the other destination I'm considering maybe Osaka or Seoul. Osaka is an area of Japan I haven't been before and Seoul is somewhere I also haven't been.
I have not been around Seoul, but five days spent early on in Osaka/Kyoto would be my choice. It would match up nicely with the 5-day Kansai Wide Area rail pass needed to explore the region good. Bear in mind it may be still rainy during September (both Hong Kong and Osaka), although less than in other SE Asian countries which get drenched at the time.
Recover and relax in easygoing Hong Kong before heading home.