Fastest way to ROI game mechanics that I have seen outside of just playing the markets.

The tax refund needs to be balanced with income so you will need to sell citizens at some point to increase income either way. I prefer not to sell young citizens, but it will become necessary for this type of strategy as the -income stat they hold will affect the daily payout due to loss of income and leaderboard status. You may be able to offset this by stacking the Economist and playing with other card combinations.