Perhaps we "buy" the house from them for the $328K balance to satisfy the lender and don't need a large down payment and then have a separate contract with them to pay $x per month until we've paid the full $172K amount.

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

The total cost for this monthly for me is $1.00 fee for online payment for Sevier County, TN and the .73 cent stamp for Blount County, TN.

Yes, the BRRRR method can work with multifamily properties, and many investors have done it successfully.

I have a W-2 Job, capital saved up for a down payment (or two), and a buy box based on my affordability.

If you haven't connected with local people in your market there are several methods here.  

Don't accept ANY partial payments.

Or a seller finance calculator showing the monthly payment based on negotiated price, interest rate, down payment, etc.

I have the opportunity to help a family member be part of a joint venture with them putting the down payment down.  

The payments and interest are high.