"If we are going to massively simplify taxes, then it needs to be done on the richest"
Why? This is a built-in assumption most people seem to be operating with and I've never understood the rationale that the rich should pay more.
Does a rich person wear roads out more than a poor one? And if they do, wouldn't it be better if they paid for how much they wear the road down? As in: a percentages of miles driven on it? As in: a consumption-based tax?
Anyhow, even if you decide to work with that flawed premise, to me, if we're going to tax revenue, then "fair" translate to a very simple mathematical concept: fixed-rate.
Everyone pays 10% tax on their income. This way, the "rich" pay more, and the "poor" pay less. Because 10% of 1M is 100k and 10% of 100k is 10k.
Fairness handled.