PFOF is so incredibly dirty that I can't believe it is legal. If you could explain it to all the voters without putting them to sleep, I am convinced most people would not support it. It may be legal but it is definitely unethical. That being said I can't see how else OP can payback YC without doing these shady things. At a dollar per user per month, even if every adult in the US joined, YC will probably shut down OP without a second thought if that was the only revenue possible.
> so people doing buy-and-hold index funds probably earn you less that way.
You will be constantly buying and selling because you need to rebalance your "index" every time the market moves... Once you go down this unethical rabbit hole, there are endless possibilities.