I've been told the hard part is finding a builder who's willing to build without me paying a large percentage upfront.

Get to know who are doing business with and never pay an upfront fee.

Here, we were after an asset we could pay off quick & cash flow aggressively.

If he doesn't make a decision - in writing - then he is required to serve the remainder of his lease, or has to pay a penalty to leave early.

I was thinking it may be closer to a "wash" or, maybe only slightly more only bc w/ a duplex at least we aren't paying for two exterior wall materials for two sfh?

Do you pay closing costs to them? 

It falls apart completely if you think of yourself as as tenant and pay yourself rent (that you would otherwise be paying someone else).https://www.richdad.com/your-house-is-not-an-assetI prefer the quote from The Richest Man in Babylon, "Make of thy dwelling a profitable investment." 

You will eventually want your nonpassive income to be flown through an S corp so you can lower the amount you pay into self employment tax.

I was happy to see you said your current bf would "pay you rent" as opposed to buying the house together.

If you’re looking for good cash flow you might want to consider properties in some of our cities as there are a lot of housing voucher clients looking for housing and the some of the programs pay reasonably well.