Since we’re techies we tend to think about technological nuances and have a certain literal frame of mind (eg “They can’t make it illegal for me to just type the wrong pin” is the type of thinking I’m talking about here) whereas in law weird precedents and your intent really matter so you really need expert advice and either way, you are throwing yourself at the mercy of a stochastic process that depends on a bunch of fallible humans along the way many of whom have the power to make your life extremely miserable.
Technicalities of destroying the data vs destroying the key to the data, destroying the data when you have a backup etc may matter a lot to us but may not mean anything in an actual judicial process depending on how it goes.
It seems to me if you have data you don’t want subject to seizure at a border it is best not to travel over the border carrying that data. If you have a backup (in your scenario), why not restore your phone from that backup after you have travelled, and not cross the border with anything that is likely to be a problem if seized? Then you’re not putting yourself at risk from this process.