My line of thought being that at least I could "lock-in" the mortgage (which I can control) at a lower cost to help keep my overall payments from getting out of control. 

Worth its weight in gold...quick and easy to install...click-lock and waterproof.

I’m considering purchasing a smart lock (WiFi enabled) for our AirBnb.

Can you share the technology that you are using - software/apps, electric lock systems, rent collection service...etc.

Iet me make sure I'm reading this right. you want to sell $6k worth of equities in one of the hottest stocks of all time, to pay down 10% of your mortgage locked in at 5.25% if that's what you're asking, then you should absolutely not do it.let your money make money. your mortgage rate is CHEAP and you'll experience zero cash flow difference from paying it down and not paying if off. 

I don't have a contract to use to lock the deal in.

A lot of times on value add properties, people will start in a bank loan, stabilize the property and then pull out money through a cash out re-fi by doing the non-recourse route and lock in for a longer fixed rate term.  

Off the top of my mind, a lease option is one method to lock in a workable price for a buyer while lowering the seller's monthly operational expense for the duration of the lease term.

You do need big capital to pull from.My house is worth about 225-250k, I'm opening Heloc for 80%, buying a property for 120k rehab expected 30k-40k expected ARV 220-240k, 6 months in refinance bought rental, pay back all Heloc from the refinanced amount since that rate is locked.

We are a very small town with one stop light and most residents probably don't lock their doors.