If you can max out your 401k or max out your IRA and then save an additional 20%+ of your after-tax, after-retirement contribution, good things really start to happen. If one is looking for earlier financial independence, such as retiring in their 40s or early 50s, it may be a good idea to skew towards more after-tax savings and investments given one has to wait until 59.5 to withdraw from their 401k or IRA penalty-free.
You don’t have to invest individually to take advantage of dividend paying stocks (i.e. investing in an ETF like DVY, which currently has a 3.16% dividend yield – almost 4%). And while your math is indeed correct, there is more to dividend paying stocks that just the math. The reason the companies pay dividends is typically because of their underlying strength, steady growth, etc. These companies can be good investments for the long run. As such, it might not make sense to sell.
Do you want to rent commercial or residential properties? I started my professional career as a commercial RE agent before starting residential investing. Buying and renting out commercial space like office, industrial and retail will generally yield a lower return but will also involve far fewer headaches. The drawback to commercial space is that it costs much more to buy one property.

If you’re looking for a way to begin gradually replacing your income, these are just some of the best ways you can do it as a physician. Remember the idea of gradual retirement? Passive income streams like the ones mentioned here are perfect ways to allow you to spend more time with family, enjoy your day job more, and, of course, make a little money while you’re at it.

Revenue sharing sites give you the opportunity to earn money online without having a website or a product to sell. If writing is your hobby, for example, then you can earn extra income by writing high quality articles and submit them to different locations on revenue sharing sites. I must admit I’ve never tried it, but I wanted to get it out as it can be a viable option to get started with.
One of the most appealing options, particularly for millennials, would be #12 on your list (create a Blog/Youtube channel). The videos can be about anything that interests you, from your daily makeup routine (with affiliate links to the products you use), recipes (what you eat each day) or as you mention, instructional videos (again with affiliate links to the products you use). Once you gain a large following and viewership, you can earn via Adsense on YouTube.
The most liquid of the private investments are investing in equity or credit hedge funds, real estate funds, and private company funds. There will usually be 6 month – 3 year lockup periods. The least liquid of the private investments are when you invest directly into private companies yourself. You might not be able to get your money out for 5-10 years, depending on the success of the company and upcoming liquidity events.
One day I thought to myself ‘This is BS! SURELY there must be other people out there in CyberLand who also need this info. I’ve spent money on subscribing to email product, purchased e-books and also downloaded lots of information which is provided by suppliers for free. However, it takes a ludicrous amount of my precious time reading the content, deciding whether it is worth saving or not, creating folders on specific subject matters, storing the data in those subject folders and it’s all eating away at my productive time. I’m NOT productive because I spend around 70% of each supposed-to-be business day just going through all this freaking content! If I packaged it up to on-sell to other people in a super-user-friendly way, surely I could make money to support myself so I can actually get on with my REAL job of building the website-with-blog I WANT to create on a subject dear to my heart?” (This subject happens to be astrology, about which I know a great deal as I have practised it professionally since 1998.)
To save time and effort, a person can group two or more of their passive activities into one larger activity, provided they form an "appropriate economic unit." When a taxpayer does this, instead of having to provide material participation in multiple activities, they only have to provide it for the activity as a whole. In addition, if a person includes multiple activities into one group and has to dispose of one of those activities, they’ve only done away with part of a larger activity as opposed to all of a smaller one. 
Venture debt ($12,240 a year): The first venture-debt fund has returned almost all my initial capital, so I decided to invest $200,000 in the second fund. I took a risk investing $150,000 in my friend's first fund, so I'm hoping there's less risk in the second fund, given he has four more years of experience on top of his 12-plus years of experience running a venture-debt portfolio for another company.
Creating passive income is the key to retiring, especially retiring early. If you want to retire early with the stock market you will have to save a huge amount of money, because that money has to last so long and retirement calculators depend on you using your investment to fund your life. With passive income you will have income coming in as long as you own the investment, without every eating away at the principle invested. Creating passive income may take more work than investing in the stock market, but I think it is well worth the extra effort!