Obviously, as your progress through life, you need to increase your reserve to match additional living expenses. 

She asked for an additional week to fix a leak in the bathroom, which I was fine with.

I would also be careful if you only have the 20% down and do not have cash reserves, things will go wrong its real estate, you need to have additional cash to ensure you can cover "what ifs."

Additionally, you may find that the FHA requirements and upfront PMI puts an undue cost on the transaction and are better off just setting up a HELOC after refinancing into the conventional on your first property, then using the HELOC to fund 10 to 15% of the next transaction.

My goal is to eventually supplement my income with additional cash flow, further diversifying my portfolio of investments, and building long-term wealth that can result in early retirement and only working out of desire vs. necessity. 

You'll be a great addition to the forums with your experience!

E.g. if my property manager would charge one month's rent in order to find a new tenant, in addition to the lost rent while the unit sits empty plus the turnover costs (painting, carpet cleaning, etc), shouldn't vacancy rates be padded even further?

Hey all we just closed on a property in Denton and I am looking into getting the process started on building out an additional dwelling unit.

Even if you have a fixed rate 30 year loan, you can pay it down more quickly by making additional principal payments. 

Additionally, if you have the cash in the bank and there is another market correction you will not have to worry about loosing access to your funding.