As a builder, I target about 15-20% profit per build ($500k-800k price range, low-mid in my area).

Fully furnished 6 month (maybe as low as it was3-4 months) is much closer to a LTR.  

Start by making a low offer to the tenants and see if they’re interested.

That is a really low rate which is good so second or heloc would be better unless you didnt have the cash to get a second mortgage and the rate difference * cost of second home dont exceed the rate difference to your primary * principle left.I would lean fixed heloc then you could always refinance if rates go down.

Some even offer preferred interest rates as low as 5.75% at no cost to you. 

I sent an invite to a prospective applicant that qualified and Looked high and low for the invite form with some additional notes to the applicant only to find out that it is not even on the platform at all, anywhere but in an email confirmation I received.

Are you looking for steady, low-risk passive income now, or are you comfortable taking on more leverage to scale faster?

I guess my question is this: is the very low cash flow (due to my low down-payment and high interest) going to improve?

House hacking is an excellent strategy, especially with the potential to use an FHA loan to secure a property with a low down payment. 

Case in point, years ago we used Olympic paints from Lowes, which were carried for a long time.