Great proposal! I learned some things about how delegation currently works just reading it 
I think one thing that might be helpful to add for readers regarding the mSOL APY not changing (please correct me if my understanding is mistaken) – the natural question is where is the extra yield that is going to Marinade coming from, and the answer is validators with 7~10% commissions (implied but never directly pointed out). With this proposal, Marinade would basically become opiniated and consider that commission to be “too high” and would attempt to normalize the yields of validators by controlling how much stake is delegated to them. I think it’s an elegant solution.