They accumulate tax-deferred allowing you to grow your wealth much faster, compared to a taxable account. 

How much do you make in your eCommerce business and therefore how much you could accumulate into the Solo 401k in what time frame?

Now, in my late 40's, I've managed to accumulate a small portfolio through very hard work.

Not only will it take forever to break even, but these examples don't take into account the accumulation of compound earnings that you are missing out on.

My duplex is near a university too, so hopefully it will continue to be in high demand as a potential exit strategy.As for my M&R, my plan was to keep $6000k aside per property for emergencies, and save 10% of rent (which is about 1.6% of the value of the property) for regular maintenance, with any leftovers accumulating for major repairs.

I'm on a steady but fairly aggressive path to accumulating a nice rental portfolio.  

I think that the partners that work so hard to accumulate large amounts of property can burn each other out and sometimes need to get away from each other to get a break from RE.

I felt like selling is not happening as quickly as I want but at the same time, I want to accumulate more rehabs while the weather is still good and we still have a few months to sell our rehab projects (before long Chicago winter comes).

You were not ready to accumulate real estate, and your strategy was flawed.  

If you want to build your net worth and build passive income, then accumulating properties that positive cash flow should be your goal.