You tell us, since you have all the answers.
The real insight I feel I have is that drug/human trafficking and other global organized crime is the reason people can't migrate freely throughout the world.
A border wall is as bad as the iron curtain, apartheid, or any other monument to national separatism; but the sad truth is that if all borders were completely opened to migration tomorrow, people would not migrate for the sake of liberty but rather to exploit the people of other nations in order to bring money back for people they regard as their own.
Colonialism was bad because people exploited foreigners instead of seeking to migrate peacefully. Today there is still exploitation that goes on because humans haven't yet grasped the concept of living sustainably and without exploitation.
Drugs/trafficking and other forms of organized crime are very exploitative. You don't get your own children hooked on drugs in order to exploit them for large amounts of money unless you are a hateful abusive parent, and yet people do this to other people's children around the world every day. As a result, some children get spoiled with expensive drug habits that exploit them for money, and by them paying out that money for the drugs, other people end up getting exploited to transport/traffic them.
If you hate the idea of the border wall and you think it will not work to stop drugs/trafficking, then start coming up with better solutions to stop drugs. When the drug markets are finished, there will be good reason to claim that borders are just a way of stopping legitimate migration. As long as criminals in one country are exploiting markets in another country, however, strengthening borders is just a way of deterring criminals in both countries from exploiting people on both sides of the border.