Well written piece, but I question the core premise. Why the fascination with maximizing “income” (passive or otherwise). Shouldn’t the goal simply be to maximize long-term after tax growth of your entire portfolio? If this takes the form of dividend paying stocks, so be it. But what if small caps are poised to outperform? What if you want to take Buffet’s or Bogle’s advice and just buy a broad market index like the S&P 500, (no matter what the dividend because you’ll just have it automatically reinvested to avoid the transaction fees).
4. Save, build and run a bread-n-breakfast place. Look at airbnb Vacation Rentals, Homes, Apartments & Rooms for Rent - Airbnb for inspiration. My wife runs one (Firdaus, Naukuchiatal) , it is not an income as of now but if you are on it for enough time, it would be, when you grow old. It doesn't have to be a fancy and glamorous thing. It could be a  2 nice-n-clean room in a city where you live. If you have a big house, it could be a  part of your house.
For a person working for a wage, their source of income is their job, or their labor (aka their time that they sell to someone). But you can own a house, and rent it out, and when your renters pay you renter every month, the source of that income check is your rental property, or your house. It would just be called rental income. If you have money in the bank in an account that pays interest, those interest payments are another source of income. We can say the source is interest payments, or we can identify the asset that is generating rent, and say your money in the bank that is generating “rent” (aka interest) is your source of income.