It has worked very well also at lowering the barrier.

I do not plan on paying interest on the HELOC for more than 2-3 months, so I guess the better question is will my portfolio benefit from acquiring a 4th property by leveraging a cash flowing LTR and therefore increasing the interest rate and lowering the cash flow on that property?

Look for loans with lower prepayment fees if you refinance or sell early.On an 8plex, a local / regional bank or credit union may have favorable terms with rates, term and fees.  

We do them, but they aren't cheap and your LTV will be lower that a HELOC on a primary residence.

Post Pandemic, the tourism and jobs continue, but at a lower number and slightly slower pace.                                                                                                                                                                                                                                                                            

Like you said with fixed you know exactly what to expect and right not fixed rate HELOCs are typically lower than Variable rate HELOCs. 

In NYC the demand is lower in the winter but the supply is also lower so apartments rent fairly quickly still (typically within a few weeks).

I have a unique property, it's a 4 bedroom home with a lower level having a few small commercial offices and then there's a 22x35 oversized garage. 

I’m considering refinancing to lower my monthly payment to around $850 and potentially using $30k-40k to invest in one of several ways.

Using a lower-cost skip tracing service and then a call/mail/sms campaign service separately may reduce your costs.