Once the rehab was done I refinanced out of the private loan into a conventional loan as a primary residence.

Did you use a hard money loan for this? 

You've got to include loan pay down, appreciation, and depreciation in your return calculations.The point of cash flow is to keep you safe.

You need two years of steady income before you can qualify for a loan.

Let's me put numbers to it: Bought a STR for $400K, 20% down, 8% DSCR loan.

Possibly using mortgage/home equity loan on one unit we own to buy an extra one.3.

The cause of the 2008-2010 Great Recession was bad real estate loans - if you could fog  a mirror, you could get a mortgage!

If I were to approach a seller of a $200k home, and they had a remaining mortgage of $100k, could I get a new bank financed loan for that $100k, and have the seller finance the remaining $100k of equity to me?   

It was 50K loan for 6 months at 20% interest.

Instead of moving into another single-family home, consider house hacking a duplex or triplex, this allows you to use low down payment loans (FHA, conventional) and offset your mortgage with rental income, accelerating your path to financial freedom.