I have no insight but I could imagine that GitHub are going to offer something similar based on that data at some point in the future.
For us the "top" developers on that page are people with 150+ repos that we depend on that I have never head of before. It turns out that they are all tiny JavaScript libraries that we depend on in some repository that's not part of our real product.
I'm happy to sponsor them as well but it'd be cents not euros per month compared to libraries that do the "heavy lifting" for us. At the moment we hand-pick things, also not ideal.
All of that text to say: This is a hard problem, I have no idea how to solve it but I applaud you and everyone who's giving it a shot.
A 5% flat fee is too much for me though, if it had a cap then it'd be different.