Now to the bad news: it is REALLY hard to be a 20yo real estate agent and make a living wage, let alone generate cash for a down payment.

.- get into real estate with a lower rate and lesser percentage down payment.

.#1 sell back to the owner with a Quit Claim deed and a 2nd position mortgage for the amount I have into the property plus interest and a down payment.#2 if the owner doesn't want to buy it back, I can begin collecting rent since I control the deed in my name at that point.  

MattIRA sells and owner finances the property for 200k and gets 30k down payment.

Your son is responsible for making payments on time or ruining this seller's credit. 

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

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. 

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

Any one of us could give you our two cents on the math of it, but only you can decide if it makes sense based on what your specific goals are. personally, i paid points on my house because my goal was lowest possible monthly payment.