What isn't addressed in this 20-year plan is the risk of the many things outside of your control: health, local economy, unexpected family members, etc.
This isn't a risk-free option, it's an attempt to minimize risk in exchange for minimal financial independence. The margin for error is very low.
If you want to achieve financial independence in a more robust manner:
1. Save up 1-3 years of living expenses. This quickly gives you the financial independence of a practically unlimited runway. It's much more useful to have financial independence early in your career.
2. Use that independence to find the best possible way to make money for you -- retrain, start a business, etc.
3. Invest. Take as much (calculated) risk as possible while you're young and become more conservative over time.
People who front-load this in their careers often (in my experience) not only retire early but do so with significant wealth.
Of course, it's your choice to make (and I definitely don't know your situation or your son's situation), but it seems to me that a big part of FI is the mindset, not the money, and I'm not sure you can acquire the mindset without going through the frugal living.
I don't think you're viewing this quite right. There's no reason you have to hit a magical 50%. Having a regular income just makes it easier to predict when you can retire / when you become financially independent.
I have also experienced an involuted perspective to money itself when on the saving mindset. Instead of seeking expansive [sic] opportunities, with an always-on savings mindset I often stop taking risks altogether. The FIRE movement, it seems, is only for those who have a reliable and predictable monthly income, or for those who can assume somewhat 100% employment till 40. If you're an entrepreneur, this arbitrary 50% saving of income won't work. You're more likely already forced to save for the next 3 months's just for wages and electric bills.