New Britain is where you are going to get the best ROI like you mentioned and this is because as a whole there are more areas within the city where qualified tenants are looking to live and the purchase prices are lower while still maintaining competitive rents.

But if you're comfortable with rougher areas, then you can definitely find properties at lower prices.

I don't charge renter's insurance for my lower class properties.

there's a tremendous demand for inventory - both primary residences and rentals - and so buyers are willing to accept lower yield than a lot of us think makes sense. 

This is my first Brrrr I'd start the process to get the appraisal ordered and try to get a tenant in there before closing on the refi, otherwise the lender may lower the max LTV

It would be a good idea to assess your tolerance to risk and then plan something like that, but using a heloc is going to be around 9-11% in this market so to pay something off with lower interest rate to borrow more might not make much sense.

These are not taxable events.

Cash out refinances after a 1031 exchange are not a taxable event.

I'm sure you could connect with students on campus who need a place to stay, lowering your chances for vacancy with the duplex you own.

If you are willing to entertain areas outside Gainesville, you should look into areas like South Fulton, Forest Park ( lots of lower priced inventory) Jonesboro, and College Park to name a few.