There is a lot of great advice and detail on the requirements for DSCR in here and one small detail that I haven't seen mentioned - You have to already have a primary residence.

I am looking at purchasing a duplex in Twinsburg, OH, as a primary residence. 

While you can't take advantage of the STR loophole if you're house hacking, you cannot have an STR at all in Denver unless it's your primary residence so it's a bit of a catch 22! 

Primary residence has about 550k in equity in Southern California.  

However 2 things to note: 1) DSCR loans for future investment property acquisitions do not care about your DTI (DTI only really matters when buying a new primary or any time you are using a Fannie/Freddie/Ginnie loan).  2) After 12 months of proof of someone else paying it for you, you may be able to omit it from your DTI calculation. 

Im wondering if i can use a heloc on my primary residence to either purchase or use for a down payment on an investment property and then refinance the investment property into a conventional 30 year mortgage to “pay myself back?”  

The primary reason a lot of people move on to bigger deals (80 units plus), is that it's possible to own larger properties in markets that make economic sense but may be far from where you live. 80 Units seems to be the target so you can hire your own full-time property manager and maintenance person, versus relying on local property managers to manage smaller multi-family properties on your behalf.

The program requires living in the property for at least one year as the primary residence.

We already own a primary residence and vacation home, so this will be seen as an investment property. 

On the other hand, considering purchasing as a primary residence, using FHA for 3.5% down, and saving cash.