Purchased a little too high at the buy (I knew it at the time too), does a little better than breaking even on cashflow, and took too long to rehab.

Consider offloading an underperformer, or a high equity property and getting better, larger or more assets.

Because of cost, duplexes are are a better choice and compete well with SFH and or apartments. 4.  

Zillow and Redfin are much better for finding listings.

Even better if you can work for a property manager or real estate compnay. 

I'm now thinking that better areas/properties make more sense because they'd likely appreciate faster (which amplifies the return) and attract better/more stable tenants. 

Are there no better options, like a REIT's, Stocks, Bitcoin, or other things?

As far as benefit to the seller goes, @Matthew Paul I think you are probably right on the money and regardless of whether they would have preferred to stay in this house rather than move, it is likely providing them some excellent cash flow; and I'd be hard-pressed to come up with a deal that serves both parties better than the current situation. 

BiggerPockets has some pro tools that let you see rent to price ratios and other cool stats, I use this to check out various markets and which seem to have better/worse profitability.

Maybe you’ll sleep better at night.