I'm struggling to generate extra lead flow from builders, mostly because it's so difficult to identify folks that are doing ground up, business purpose construction. 

Purchased and sold on the same day, it generated a $50,000 profit.

If I assume this is a rental property, your income isn't over $150k, and you're not a real estate professional then the losses the rental property generates could offset other income up to a certain amount. 

A single property is simpler to manage, carries lower financial risk, and offers streamlined tax reporting but may generate taxable income sooner.

House hacking lets you live in one unit and rent out the others, which can help cover your mortgage and generate some extra income.

Here are a couple of developers you should take a look at Trion Properties, their principal Max Sharkansky posts here on BP, https://www.biggerpockets.com/blog/top-5-markets-multifamily-rentals-2019and Feldman Equities out of Tampa (3rd generation real estate family famous for having built the terminals at Chicago O'Hare) who are among the only ones opening up downtown skyrise office buildings to accredited investors.

The ability to generate more income for simply the cost of a building is another huge cost saving over buying an expensive commercial lot and developing it.

I think the HELOC is the way to go if you can - However after reading your post, if you are talking about a 6 figure pay day, how long would the property take to generate that 6 figures in cash flow?

She bought a duplex in San Antonio, and with a solid property manager, it’s been a consistant income generator for her.

I simply don't understand the point of SubTo when there is a many generations old existing legal instrument readily available for use.