I personally don't see any benefit of doing HSR on that route as it is slower and generally no cheaper than flying.

Biz would be ok for that distance. 1st is debatable. 2nd class is a no no for me under any circumstances.

Possibly more important in your case, as has already been alluded to, there is nothing particularly memorable or romantic about the HSR.

The slow sleepers have more character about them though again for me sharing a sleeping space with strangers is a no no. The HK - BJ train has 2 birth compartments with en suite and is a really nice trip. Can't remember if that train stopped at GZ though.