Sadly, Hong Kong has changed since the Covid hysteria: it is no longer a late night place and is pretty dead after 10pm. Yes there are some bars open late-ish, but you wouldn't get in to town until maybe 2am, so all you'd find would be a very few bars and some local all night eating places.

Your options are basically the Regal Airport Hotel (the only one walkable from arrivals) or hanging around at the airport, using lounges when available. Note that the lounges are in the departure area and once you've gone through transit security to the departures level it is very difficult/impossible to get back out and enter Hong Kong.

Personally I'd probably change to an earlier flight from SIN if possible and have a proper evening in HK.