The articles you are reading on google normally skip a lot of important factors to get to the benefits of a Series LLC. 

If they come in as partners, they can deduct the interest expenses directly against their share of income from partnership.If they come in as lender, they can still deduct the interest as investment interest but might be limited depending on various other factors such as need to itemizing deductions rather than standard deduction. 

Another wrinkle is that I was debating whether it'd be a good idea to get a HELOC on our house as another source of capital, but would that also be factored into my DTI?

It’s never been a factor to me. 

There are a lot of factors to consider, but it's generally worth it.

Opportunity cost is definitely something to factor in as well.

Also, make sure to factor in the implications of future inflation on your assets. 

This is without factoring in the cost of hiring a property manager (I do include mello-roos, solar costs, and ~10% vacancy rate - jeez these are expensive). 

DTI is an important factor when determining how much house you'll be able to afford.

@Cory LucasLots of factors... which has the higher interest rate?