Honestly, that wouldn’t be a bad way to fund education: education is free, but the university gets taxation power over you so they can tax you at x% of your income. It aligns incentives better than the current system.
An actual graduate tax would be far less regressive than the current system
That would mean people that get great paying jobs right out of college would pay more than they even borrowed, but it would be justified because the degree would likely have had a big impact if it was so soon after finishing the degree.