Relations will be as good with Mexico, as Mexico wants them to be. Live within the laws of good neighbors, and you’ll have a good neighbor. Don’t, and you’re buying off more than you can chew.
Enforce your own borders. Keep your citizens in check.
Be good to your citizens, so they don’t have to flee for prosperity.
Mexico could be a much much better place.
Something like 2% of their economy is remittances from the US.
Plenty of money to build the Wall.