Neville, the problem with “secular” midnight is not that it is goyish, but it is not based on any physical event.
Hatzos is defined by the time between sunrise and sunset. As we see with DST, midnight is totally a human convention. For example, if you have two cities nearby in different time zones, as often happens in US when they are in different states – do their mazalos differ by an hour? And whole China is one big time zone – so their 6th hour could be in the middle of the night or day.
So, you are trying to be traditional and follow a minhag and then tie it to a totally artificial convention.
As to the main sugya, it may be that both following and ignoring mazalos have respectable traditions. For example, gemorah has a lot to say about amulets (but provides for an empirical procedure to test their efficacy), while Rambam rejects them. We could apply Pascal’s wager here (if you are an agnostic – not sure that G-d exists, it is less risky to be frum) and follow mazalos, unless you can encounter a large expense, such as paying for an extra hour for the wedding hall. For some, just having their soup cold on Shabbos evening outweighs other considerations.