I was thinking about an strategy of doing Cash Out Refinicing putting that money in the stock market (distributed in ETF and some "safer" stocks) to get around a 6% anual return (estimating that low because is the "standard" average, however my current stock portafolio has a cummulative return of 66% in the last 4 years).

Of course not all of these are exactly like this but I do see quite a bit of the same choices still.

We don't do an 80/20 or 70/30 rule as it doesn't give you any ground to stand on if ever audited. 

If there are common areas in the commercial property, then sure you would need to do the accounting regarding who pays what and what's collected among the various tenants. 

Is this a common thing most DSCR lenders can do? 

Find out how to do this first.

Or do you need cash and therefore looking at doing a HELOC to get it?

The time to do the reverse is when people using low down payments (ie 5%) to buy it and loans that require you live in it. 

Are any veterans already doing this that might suggest elsewhere ?

The sad part is that these poor returns don't even have to do with financing costs which will be an issue in the future this is simply poor performance by inexperienced operators.