Without leverage there are more passive means to obtain similar return with similar risk. - adding ADU is significant effort.

I prefer holding a rental to mitigate capital gains taxes after a few years of holding.

I highly recommend that you attend local REI networking groups and meetups to connect with fellow real estate investors and check out the forums, you can find discussions specific to your goals there and gain valuable insights.

I would consider a blanket and portfolio loan as you gain more properties. 

Flip is another word that people use to mean sell but after it was held as a long-term investment.The intent is important here as it will determine whether the sale will be eligible for a 1031 exchange or if the gain can be eligible for capital gain treatment.Best of luck.

Leasing a LTR is classified as passive despite the occasional needs to actively protect the asset.In Ocean City, NJ there was a little store that had stores within it.

They are still a marketplace, but are working hard on transparency, data aggregation and reporting for buyers and trying to offer a nice alternative for investors who want to go the passive route.  

For example, if I were to purchase a home for $1M, fix it up then sell it for $1.4M, how much of that $400k gain would be taxed?

Now I eat anything I want and have trouble gaining a pound.

If that is the case, then you will be paying on the capital gains taxes of the sale.