Apply 5% tariff.
Review Mexico’s plan(s) to halt illegal crossings.
Quantify drastic reduction in illegal crossings for several months.
Only then pull the tariff.
If illegal crossings thereafter increase by more than x%, there’s a 10% tariff for 6 months, and crossings must decrease.
Rinse and repeat with 15% next time.
Give bounties for identifying illegals and employers of illegals.
Deport all illegals.
Fine the crap out of those law-breaking employers.
Deporting illegals will come only after we close the border.