Why would you do HELOCs and not just regular mortgages? HELOCs have variable rates and short terms.
Most banks will do conventional mortgages on up to a total of four mortgaged properties. A few will go up to ten. Beyond that you’re looking at some sort of commerical loan. You may be able to do that on the first few properties, too. Only a few banks do this sort of loan, but the ones that do will be more investor friendly than conventional lenders.
Realize that if you want to use a new appraisal for the value and you want cash then you’re looking at owning for a year. If you’re willing to use the price you paid as the value, you can do this quickly. Not sure why you wouldn’t just get the loan up front in that case.
You’re about five years too late to make this strategy work really well.