@Zachary C.
Great question, as always.
I think your research has most of the repayment options covered so I will offer another take on the problem at hand.
While your scenario offers three aspects of the loans (balance, rate, payment), the solutions only address two of them:
Avalanche (rate) and Snowball (balance).
Given your stated premise of maximizing cash flow (through efficient debt repayment), I would characterize the retirement of monthly payment obligations to be the primary goal.
From there the questions becomes: What is the quickest and cheapest road to achieving that goal?
In this case, paying off C first appears to be the optimal combination of quick (low balance) and impactful (high monthly payment retired). I believe these two elements outweigh the lower rate.
The process happens to align with the snowball method in this particular situation.
Time is money and money is time. When you choose which loan to repay first, you are essentially borrowing from the other three loans to do so. The price paid is the weighted-average rate of the three. This is why i would consider paying off A last.
B and D are much more equal with a slight favoring of D's lower balance over B's slightly higher rate and monthly payment.
Of course, caveat: ESID.
