Another example of a good use for a HELOC would be a HELOC against a primary residence to use for doing flips.

I recently bought my first investment property that will start as my primary residence with 5% down.

If you move into the ADU, do so prior to placing someone in the primary.  

Hi to all I am a brand new to this BP and a completely beginner investing in real estate.I have applied for a second mortgage for my next primary residence and on the process hopefully closing at the end of August (brand new home), now I have contacted some credit union to apply a HELOC and advised me to apply before closing of my current primary home.

-Thinking long term, is this something I could scale to make my primary source of income?

It sounds like you want to buy a primary residence.

Also, DSCR lenders have lowered cash out loan to value guidelines, so cash out to 80% LTV is very hard to do.Personally, I think the best way to add real estate is if you can buy a new primary home every 1-2 years. 

But I'm also building an ADU here at my primary in California.

I am looking into selling our rental property and potentially our primary residence.

Originally, my lender said there wouldn't be an issue financing the home as a primary residence because I am on the deed and will be living there.