I think you need to focus on one strategy first before you explore too many different options.

A detailed screening process and ongoing training can make a huge difference in performance.• Clear Guidelines & Performance Metrics:Defining clear expectations, call scripts, and performance benchmarks helps maintain consistency.

If you feel ambitious, you could try a BRRR strategy adding additional equity to the subject home and then taking equity out of it to pay off either the HELOC or if you decided to the refinance the primary residence. 

People had to move to used  or as we did had to open more services to match different box configurations.  

If meaningful appreciation is on the horizon, its a different story.If you were to go the financing route, blanket loan may be your best bet through small local bank or alternative lender.

Just make sure you take your policy to the brokers so they can do a direct comparison.Maybe hit up your primary home insurer to see if they offer a STR policy.

These are very different than Subject-To, as the loan actually transfers to your son's name, and off the seller's name. 

It also gets treated differently in Texas vs CA vs AZ.

You've got some different comments above so let me help clarify some things if you don't mind.1.

I considered sound proofing the ceiling of the unit below but I’m reading articles saying that it can be very pricey and it won’t make a major difference.