It is so much better when you properly explain it as an operational problem rather than a theoretical exercise 🙂
At country border, your method is good : find clean frequencies. If this cell’s neighbours are using SFH, then I would recommend to remove the “clean frequencies” from their list, so that your NH cell will not be interfered by your SFH cells.
Once you are happy with your NH cell, then apply BBH and don’t look back 🙂
One last thing : try to find an agreement with the operators from the other country, so that they won’t use these clean frequencies ever at the border. (For sure, they are facing the same kind of issues than you are, they will like to ensure few clean frequencies for themselves)