If you can qualify for conventional cash out refinancing, most lenders do not have a minimum loan amount requirement.

3.5% down conventional What was the outcome?

Also for conventional financing, the DP can't be borrowed.

I would find a loan officer that does both conventional and FHA renovation loans to compare which has better rates and terms for your situation.

A DSCR loan sounds like a great option for you based on the fact that you already own five properties which I presume have conventional mortgages on them, based on your post. 

The plan was to utilize the HELOC as the down payment then getting a conventional loan to cover the remaining 80%.

Instead of moving into another single-family home, consider house hacking a duplex or triplex, this allows you to use low down payment loans (FHA, conventional) and offset your mortgage with rental income, accelerating your path to financial freedom.

Also, HomeReady/HomePossible (Conventional loans) can be just as favorable for househacking but also bring Conventional Conforming income guidelines to the deal, rather than the more restrictive FHA guidelines. 

You can finance it with an FHA loan (3.5% down) or a conventional loan (5% down) if you plan to live in the property.

I was wondering between FHA and conventional.