Before I begin with how to fix this... We do not have 'radically low taxes'.
If you judge in the context of wealthy nations generally, we definitely do. Our tax revenues are lower as a share of GDP than any other large, wealthy nation's. There are small "boutique" economies with even lower taxes as a share of GDP, but that's largely because of GDP being inflated by way of accounting quirks (basically, banking/finance hubs where companies account for gains actually produced elsewhere, creating a misleading ratio). Yet, even some of those have higher tax revenues as a share of GDP than us, like Switzerland. No big wealthy nation has taxes anywhere near as low as ours.
Replace it with a Flat tax of 20% on earned income from 0-$500k. Additional 10% tax on anything over that.
That would be awesome news for the super-rich, who would experience it as a windfall. The most recent data I saw says the top 1% paid an effective rate of 33.3%. If you made it so it could never rise above 30%, they'd be much better off. So, if you think our problem in this country is that the rich haven't been pulling away from the rest of the nation fast enough: problem solved.
Meanwhile, your 20% tax, even with the $30k standard deduction, would be a tax hike for the middle class.
https://www.taxpolicycenter.org/statistics/historical-average-federal-tax-rates-all-households
So, again, if the problem in this country is that the middle class simply has way too much take-home pay: problem solved.
Now, what do you think that new income tax structure would bring in, in terms of revenues? Keep in mind the existing tax, according to that link I provided, averages 21.1% for the population as a whole. Throw in a $30k deduction for everyone, and you're going to be coming in with much lower revenues than currently collected, right? So doesn't this move you in the exact opposite direction -- towards larger deficits?
3) Healthcare costs: It doesn't matter how we pay for healthcare if we don't address the cost issue. What is driving the costs so high? Why is it so expensive to get a medical degree? Why are insurance costs so high for doctors/hospitals? I have more questions than answers here, but this is first place to start.
We have plenty of models to work with, in terms of systems that achieve better public health results for half the per-capita cost that we pay. I'd start there. What would we need to emulate those systems?
4) Social security: Fix it with an additional 3% tax on income over $500k. That would resolve the issue. Social Security is not nearly as big of a problem as people think. Medicare is... which is due to number three above.
Even just taking out the SS tax cap would do it, for the foreseeable future. So would including unearned income (dividends, capital gains, interest) in the SS tax base. I agree with you that Medicare is the bigger issue.
I think the area for discussion for the two of us would be on taxation. I want more transparency and simplicity in the tax code.
I do, too. But you don't need a flat tax for that. The brackets aren't what take up so many pages. You could fit a highly progressive tax into a single line of text, if you used a polynomial equation, or into a half-page of text, if you used a bracketing system. The complexity comes from sweetheart deals.... for example, special treatment for farms as opposed to other businesses. Once you start creating those sweetheart deals, you then end up growing hundreds of pages of text to keep those deals from having unintended consequences. For example, if debt on farms gets treated better than debt on other businesses, for tax purposes, then you need all sorts of rules to make sure people aren't calling things "farms" that aren't really farms.