If I were designing a system, I'd have it staggered, so something like:

February states: 20% of delegates go to state winner; rest are proportional with a 15% qualifying theshold.
March: 40% winner take all
April: 60% WTA
May: 80% WTA
June: 100% WTA

— Nate Silver (@NateSilver538) April 14, 2019