Make sure you know the value the ADU will add to the property before building the ADU. 2) the financing on an ADU is typically far worse than for initial investment property acquisition or is often not leveraged by the ADU (HELOC, cash out refi, etc).

DSCR Loans in second position is not a thing - you might find a DSCR Lender that have other products that fit this need, like a Closed End Second or HELOC, but they aren't really "DSCR Loans" - I think you are likely looking for a CES (it is true however that you are correct in looking at the same place though, most of the lenders offering this on rentals are also DSCR Lenders)

Please help me find a bank, credit union, etc that will let me use 100% of my equity on an investment property for an interest only HELOC.

Leaning towards a home equity loan or refi as choices since I've been turn down for Helocs due to DTI ratio which i find strange.

My recommendation, house hack, then move on, and then consider selling with a 1031 Exchange, take out HELOCs, etc. to build the portfolio.

I have started going down the road to do a DSCR cash out refi on a duplex (BRRR) I purchased last year with my HELOC, but not that far yet.

Besides traditional banks to access the equity via HELOC or refinance, what other programs are there via private lenders that will allow me to access that equity?

Hopefully, I can get a HELOC on this later.

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

Sounds like you should be shopping for a HELOC that you can tap for investment money.