Once you've decided that, if active, find a deal that makes sense and do it, IMO, multifamily is more scalable than SFH. 

My questions: 1) Does the above strategy make sense, and can it be profitable & scalable?

Again, I want something scalable to be able to apply to multiple properties quickly and effectively. 

Having had some experience with this strategy since 2018 which has allowed me to retire from my w2 with cash flow from them, I am looking to streamline into a scalable business, while I educate myself on commercial residential investing.

I know there is a lot of focus on using those "golden ticket" conventional loans, but once you jump into multi units it is just so much more scalable.

-How scalable is this strategy?

That makes it very scalable.

Has anyone ever opened multiple lines of personal credit, then used that money to purchase the property, and then cash out refinanced to pay off the lines? If so, what was your threshold (margin of safety) to execute ...

It is not scalable or sustainable. 

Typically the larger or more units you can acquire the better the deal can perform because you have scalability to run the property like a business