As a homeowner turned renter and retiree from another state, I had assets (savings account balance) to prove my capacity to pay the rent for a year and a good credit score to prove my character of paying my bills on time.

Your credit will take a massive hit which may not be worth the interest savings of 18months compared with potentially worse terms on a new mortgage down the road (some lenders give rate discounts tiered with credit scores).I did exactly what you are proposing and have been wondering if I should just pay it all back. 

It's only roughly $100 "per door", but our goal overall is total cash flow, so $350-$400 total after all expenses and saving for repairs seems good to us.

Again more work for you, but you’ll save at least a few thousand just by doing that.

Being able to close in 30 days or even sooner is more valuable than saving a fraction of a point on the interest rate, in my opinion.  

I’d like to pay low or no money down to save capital for the next deal & perhaps a few small renovations.

We find it best to have a checking account (for operations), savings account (for capital expenses), and a money market account (to keep security deposits).

I have my eye on a few properties for now and I'm about halfway to my capital goal, but the money can't seem to get saved quickly enough..lol.

Have saved up about $100k+, could be more if I sell off some stock, and am looking at the Bronx as a market (maybe nearby NJ, but have heard it can be tough with property tax so high in NJ).

Ideally we wouldn’t buy if it didn’t cash flow in the first place so I’m picking nice round numbers just for simplicity.Loan is 240K 30 yr. fixed at 4.3%Monthly payments are 1,500 Rental income 2,000Cash flow is 500/monthIf we use the cash flow to pay down the mortgage faster we can pay down the mortgage in ~ 17 years and save nearly 80K in interest paid on that loan.