US people have to get a second citizenship and renounce their US citizenship before they can achieve the tax rates you mention.

Not true, there is an exception for income under 80k/year. Over that person can operate via LLC with dividends etc. Most people renounce because it is difficult and frustrating to deal with US tax system.

Self employment and social security will still take 15%

Can you name a few such countries. I live in the EU, but when you have a business in my country you are always breaking some rules no matter what, and a lot is up to the discretion of the tax people.

Romania. 10% income tax, 5% dividend tax, 1% corporate taxes for startups up to 1m revenue.

Cost of living: very low, 30-50% less than Austria/Germany.

That's interesting. Do you have nay more details or link to resources on Romania?

There are some comparisons, like https://thebanks.eu/compare-countries-by-tax-rates

But a lot of details are missing in them. Countries like Romania, Ukraine and Georgia have single-digit % tax for sole proprietors, but IIUC it applies on revenue, not income.

Estonia has 0% corporate tax, but 20% on dividends, which should go well with technique discussed in the OP.

Or for the higher income brackets, Italy has €100k/year flat tax: https://www.economist.com/europe/2020/10/29/a-flat-tax-schem...

For self employed devs Slovenia has 80% expense flat rate, so final tax is like 10% under 100k with zero administration. Bulgaria has LLCs with 10% income tax. If you have to buy property to get residency, there is Greece, Cyprus, Portugal...