I could pay a regular marketing firm to build a site, but since so many STR owners have this need, I have to imagine there's an off the shelf option rather than having a custom site built.

Pay your house off then only buy investment properties in cash. 

I replaced a furnace at a rental with in base model, I don't recall the BTU or efficiency off the top of my head, but it was $3450 so I would guess you are going to be upwards of $6500-8K depending on sizes. 

Given your goals, the main drawback with these is that you cannot live in the property, so house hacking is off the table. 

Update: It did not help.We're going to either have to sell or refi to get her off the mortgage or I'll have to purchase investment properties solo.

Turns out inexpensive kitchen updates pay off big time when you sell your home, with homeowners recouping an average of 85% of their investment.6 updates you can make to your kitchen without renovatingDid you know that a minor kitchen renovation project costs around $22,000 and a major kitchen remodel can set homeowners back $65,000?

I don't think procuring a new mortgage would be beneficial unless you need to pay off the cards.

30 year would generate cash flow quicker for sure, but it would obviously take longer to pay off the property.

I had previously used LendingHome as my HML but they stopped lending in Illinois, I asked a local investor @Mark Ainley who referred me to the Eric Workman the VP of lending for Renovo Financial, they are local (Chicago area), met with him and felt we hit it off well.

Basically the bill just gets stuck on the other homeowners account with interest accruing and at some point in time they try to sell or refinance the unit and it needs to be paid off.