But you are forgetting that there is a reason why saying Gut Shabbos can be considered Kiddush. It’s not magic.
When you say Gut Shabbos, on Shabbos, you are acknowleging the Kedushas Hayom. When you are actually not acknowledging anything, then it is not Kiddush.
What would you say about someone who tells his son, “This is how we will make Kiddush when it becomes Shabbos, Vayechulu…”, do the magic words make it Shabbos? What about having this very conversation on Erev Shabbos, will you say the Shin word?