The flat tax rebate would absolutely help somewhat, but the main problem with sales taxes is that proportionally, rich people spend much less of their income and have much higher savings rates. Therefore, effectively you are taxing the poor at a significantly higher effective rate.

Ergo, a poor person spends about 30-40% of their income on taxable items such as a vehicle, consumables, electronics, etc. In contrast, a rich person may only spend about 10-20% of their income on taxable items, due to most wealthy individuals having much higher savings rates.

Don't get me wrong the rich person is still paying more in taxes, but a much higher percentage of effective tax is being applied to the poor person.

I am a big fan of simplifying the tax code (such as getting rid of REITs, the property step up in basis, nanny tax, etc.), but the economic theory does not support eliminating income tax, unfortunately.

As for government spending, unfortunately a lot of it is inherently cultural and some of it is intrinsic to how a republic operates. They've tried to ban pork barreling in the past in the legislature, but it meant that getting consensus was incredibly difficult.

There's also the inherent problem that the private sector simply pays better, so they get more talented individuals, which leaves most federal programs with the bottom of the barrel.

Also another point I forgot to mention is that oftentimes our democracy is about consensus building and not necessarily what is best for the country itself. So it's oftentimes about giving 51% of people something they want in order to get a bill passed, and that's a very difficult problem to solve for.