The maximum fixed-rate period seems to be seven years, and there is generally a balloon payment at 10 or 15 years.

I used a HELOC to cover the down payment, appraisal gap, and furniture, allowing me to enter the STR market without depleting cash reserves.

I’m close to saving up for a down payment but it’s taking forever!

What kind of account or vehicle would you suggest to start saving down payment money for my first rental property?

I do not understand your remark as related to the DSCR loan.  2 ADUs are not allowed by f/f conventional financing but is not always enforced.Fortunately if you stay with the current tenant without a refi, by the time you pay principle it is likely the rent will have increased enough to absorb the increase mortgage payment. 

However if you have enough reserves, 6mo or greater then I'd say chance it but a couple of missed payments from the tenants and you have to foot the payments out of your own pocket.I'm all for taking risks but I think you really need to evaluate the age of the major systems so you don't get hit with a bunch of cap ex all at once as well as your reserves. 

Still use paper method for mileage logs.Tried QuickBooks several years ago but it was more complicated and I did not need the bells and whistles.

These are features like online renter payments, tenant portal (for payments and work orders), help keeping track of work orders, tax reporting features, and accounting functionality.

We just need to verify that you have cash for the downpayment, closing cost, and up to 6 months of the mortgage payment as reserves.

In addition to taking 6-9 months to close, they are also subject to (i) annual audits, (ii) annual REAC property inspections, (iii) recurring annual MIP payments on top of your interest rate, (iv) large initial deposits to the replacement reserve, (v) commercial space limitations to 15-20% EGI / 25% NRA, and (vi) less forgiving underwriting with 7.00% minimum vacancy rates when determining our underwritten NOI.