I work with many PhDs so I’m familiar with the problems. But I don’t know enough to have a solution.

It may just be “pay them a living wage!”. Pay them $75,000 and call it a day. The waived tuition cost is BS. They aren’t undergrads living on campus and taking a half dozen classes. It doesn’t cost $50,000 / year to have a PhD student do lots of work for the school. Quite the opposite! The school makes a lot of money of their labor!!

At least for STEM PhDs at research universities.