I’m been thinking instead of having fixed “Income Tax” and “Flat Tax” options, what about a single Income Tax, and an uncancellable slider which sets ratio of low income tax rate:high income tax rate? That way you can choose how progressive taxation is, rather than just having a single percentage slider.
This would mean the “Income Tax” policy value reflects “Middle Income tax”, with the other slider altering High and Low tax rates.
So for example, you could set “Income Tax” to say “20%”, and the “Income Tax Ratio” to 0.25, meaning the tax rates would be “10%”, “20%”, “40%”.