Buying more properties, if buying correctly, simply does NOT increase your DTI or at least in my example above much at all. 

Death Rate: The death rate has been gradually increasing.

They had an unfinished space counted as finished space increasing the size of the home by ~10%.

Just make sure you are saving towards those slow months during the peak months, buy a deal at the right price (don't overpay just because its a turnkey STR or you think it will make more money than projected), and look into ways to increase bookings during those slow months with some marketing and operation tweaks. 

Without actually building the 3rd unit, would this new triplex eligibility increase my property's value? 

You could consider the areas in the city with the highest value increases.

As they see their money is safe and you're succeeding with your plans, they'll usually ask to increase the amount of money they loan to you.

They further out the ripples go from the stone, the less appreciation your receive while the cash flow increases.

You will then naturally refine your approach as you get feedback from results.

While less than 10% of the mortgage industry offers 40-year mortgages, here's something to factor in: they often come in with a rate increase of 20-35 bps.Assume 35 bps.$300k mortgage at 7.25% (30-year): $2,046$300k mortgage at 7.6% (40-year): $1,996Technically, it lowers the monthly, but not by much once factoring in the rate adjustment.