Goal would be to acquire 20 rentals by the age of retirement, with some already starting to be paid off.I am in the same boat as you, and I focus on safe appreciation all day (small cash flow for safety and then focused on markets on that appreciate). 

Currently researching the best markets to focus on and would love to hear insights from others on whats working for them.

I don't know about you but I am constantly on the hunt for consistency, consistency of income, of deals, of productivity, in my relationships...so that's my focus in all things not just real estate.So my vision to your question is a a state of total consistency of income and deals.I'm focused on the basics, which to drive me is even more specifically "making offers."  

The only reason I recommend focusing on a couple neighborhoods, is because house hacking entails buying in a neighborhood you want to live in.  

My focus is simple: helping out-of-state investors like you unlock the full potential of the Birmingham market.

I assist my clients with turnkey opportunities in DFW and other TX markets, if that is something you want to find out more about I'd love to share some thoughts on areas within Dallas to focus on.  

Make sure they feel heard and assure them you will do in your power what you can do to make sure they are safe.

That approach makes a lot of sense—focusing on key value-adds like bathrooms, curb appeal, and open layouts can really improve returns.

My suggestion would be to consider the properties you're purchasing and focusing more on ways you can drive up the value of each property more quickly.

Napoleon Hill's masterpiece takes a dive into the power of the mind and the principles that lead to success.