you mention you are overseas quite a bit, but maybe you could still have a primary residence with a primary residence loan.

Down payment can be higher than primary, but the good thing is that you won't need that much income to qualify because lenders can you 75% of the market rents for the units of the property.

If I buy a property for short term rental and do a cost segregation study to speed up depreciation, can I later decide to convert to a primary residence?

Are they putting down money as a down payment and do they have a primary residence they realize they could also lose if they do not pay you (depends on state). 

I have tried going through the conventional route of HELOC and Home Equity Loan for my primary residence.

Who is your primary customer, what is your RevPAR, what financing terms are you looking at, who is running the daily operations, are you looking to get under a major flag, i.e.

Get the primary under your belt.

That being said, Zillow is also the primary site in my market for rental searches, especially SFRs, so I have no shortage of tenants seeing my listing when it is available.The one downside, which is minimal, is even though the tenant pays rent through Zillow on first, and I get a notice that rent was paid, it can take 5-ish days to arrive in my bank account.  

It seems in order of increasing profitability:LTR < MTR < STRGiven that I'm based in the Denver metro area, unless I'm occupying the property as my primary residence, STR is out the question, as Denver has cracked down on the short tern rental laws.

I currently own a duplex and a sfh (my primary resi) on the south side of Chicago.