A loan-to-value (LTV) ratio around 70-80% with a straightforward approval process would be appealing.

Get a few quotes and confirm you’re comparing the same coverage.

You could probably do a NPV analysis on that compared to the price of installing a water softener up front.

Very comparative to purchasing something myself, yet total freedom.  

I would use the Find a Lender tab on here to reach out to a few lenders and compare. 

I financed this deal through a private lender, allowing me flexibility and faster access to funds compared to traditional financing.

BUT, once we applied for the card, we were rejected immediately because our debt is so large compared to our income because DUH, we are real estate investors.I figured a lot of other investors might have run up against this issue and I was wondering if anyone figured out how to work around it.

On that website shows a lot different price compared to Zillow. 

You're also LIVING there when you house hack, so you have to factor in what your "normal" comparable living costs would be. 

Anything not actually apart of your main house (or a basement that is below grade) is considered Additional Square Footage and will have a fraction of the value per sqft as compared to GLA.