I've been interested in this type of investment and I'm guessing this is sort of different, since it's not secured by real estate which seems like the more traditional way to do it. 

I truly cant think of a reason why I would ever go back to traditional renting 

Additionally, you have many borrowers that do not qualify for your traditional A-paper mortgage from a bank and that will definitely not help, as they won't even have the opportunity to buy.

I do however, have full availability on my HELOC, various credit cards that are offering me 0% balance transfer checks for up to 12 months, and various retirement funds (Roth IRA, traditional 401k, etc.)I'm already a homeowner in SoCal, but I want to buy my first LTR SFR someplace like Cleveland since the price point is attractive.

I would never do a commercial loan with shorter amortization and balloon payment. yes lower rates for now, but you have to do a refi at some point and if you do all of your loans on shorter terms, you will not be able to access as much traditional financing potentially.

I’m considering traditional lenders, but open to creative options.If I were to bring on an equity partner, what’s a fair and motivating structure for both sides?

You can then avoid hard money lenders and go with a traditional bank.

Still retired from a traditional W2 job, but working harder than ever! 

Financed through two traditional 30 year mortgages.

But just trying to see if there are better ideas then taking traditional way around.