At the moment, income tax is simply positioned as a progressive tax, that affects all income levels, I assume to a fixed degree.
In reality, the extent to which the poor are effected would be impacted by the income threshold at which you start paying income tax, and the rate at which that kicks in.
For middle income, the tax free allowance would be a factor, but the level of the first tax bracket and the point at which the second one kicks in would be more impactful, as well as the % of that second tax bracket.
For the wealthy, the only thing that would really matter would be the % of the top bracket. In the UK, it reached 50% after 2008 and was dropped to 45%, as allegedly that makes HMRC more money as it pushes high earners away less.
My proposal then would for the income tax to have three brackets, that primarily (but not exclusively) affect poor / middle income / wealthy, with the level of each being adjustable.
For each bracket, a separate percentage can be defined too, for the tax level of that bracket.
If it’s easier from a dev perspective, could the current income tax just be introduced as 3 different policies, one corresponding to each band and you can cancel the policy if you don’t even want a middle band, for example?