They have some things designed for higher end investors.

Ironically, platforms designed for real estate (Zillow, Apartments.com, Rentler, etc.) produce very few tenants for me, compared to FB Marketplace (and I also find a lot of their functionality to be user un-friendly, from the landlord side). ...you'd think that platforms custom-made for real estate would make the process a snap for landlords, but it takes me a lot longer to post and manage rental listings on Zillow or Rentler than it did on Craigslist.

Since you've lived there for nearly 3 years, the fact that you made the effort to contact the owner may be enough to move the needle.

Following this, I consider it another pro of our situation that I have the ability to do significant property renovations myself, and between my wife and I, we make a pretty good design team.

BiggerPockets is full of resources designed to help you along your journey, whether you are just getting started or a seasoned investor.

., how much design work does the civil engineer need to do. 

Congrats - love the design

I have a general contractors and interior design license.

He received quotes to remove the texture ceiling, paint the interior, purchase and install new kitchen cabinets and counters (including completing the design), picked out 2 floorings to get quotes which will be provided this week, and determined furnace is beyond saving (I will likely get HVAC quote for him).  

Supply of STR units: We have seen developers in Tulum building cookie-cutter projects with a high density and cheap design/finishes, these units are not renting that well and some owners have switched to long-term rentals, which were very expensive and scarce.