Aiden, I think I'd back up one step before deciding whether DSCR, an ARM, interest-only or seller financing is "best."
If you're serious about building a rental portfolio, the first question is what is going to be the basis for your borrowing as you grow.
For somebody starting out, one of the best opportunities can still be owner-occupied multifamily. Buy a duplex, triplex or fourplex, live in one unit and rent the others. Depending on the circumstances, FHA or conventional owner-occupied financing can let you get started with considerably less capital than buying a property strictly as an investor.
Live there as required, build equity, save money and learn how to operate the property. Later, when you move into your next home, the original property becomes part of the rental portfolio.
As the portfolio grows, though, I generally like investment properties to qualify based upon the economics of the property rather than continually depending upon the investor's personal income.
That's where DSCR financing becomes an important tool.
If I'm buying a rental property, I want the success of that financing to be based primarily on whether the property produces enough rent to support its debt. I don't necessarily want every investment property tied back to how much money I make at my job or in my business.
Once you've established that strategy, then you can start looking at the different ways to structure the debt.
Sometimes a fully amortizing fixed-rate DSCR loan works perfectly well.
Sometimes interest-only makes sense because lowering the payment improves cash flow or gets the property to the required DSCR. But remember what you've done. You've improved today's cash flow by not amortizing principal. That can be a legitimate strategy, but it isn't free money.
An adjustable-rate loan can make sense too, particularly if the initial rate and payment are meaningfully better than the fixed-rate alternative and you understand exactly when and how the rate can adjust. Then you need an exit strategy if rates don't do what you hope they'll do.
Seller financing can be terrific when the opportunity presents itself because now you can potentially negotiate the rate, amortization, down payment and balloon instead of accepting a lender's standard structure. But again, the property still has to make economic sense.
That's really where I'd be careful with the phrase "make the numbers work."
Financing can improve a good deal.
It shouldn't be used to disguise a bad one.
If a property only cash flows because I use interest-only financing, assume rents will increase, ignore future capital expenditures and hope I can refinance into a lower rate three years from now, I probably haven't solved the problem. I've postponed it.
I'd underwrite the property first and then compare several financing structures against the same deal.
What does it look like with fixed-rate amortizing debt?
What does it look like interest-only?
What does an ARM actually save me?
Is seller financing available?
How much cash am I putting into the transaction and what reserves do I have left afterward?
And most importantly, what happens if rents don't increase and my expenses do?
I've spent a large part of my career financing and owning income-producing real estate, and I've always looked at leverage as a tool, not the objective.
The objective is owning an asset whose economics work.
The financing should help you accomplish that without putting you in a position where everything has to go right for the investment to survive.