Or would I be better off keeping it since it’s cash flowing (been this way since day one of renting it 4 years ago), saving some more of my own cash again, and just investing that instead into a short term rental property?

I just don't allow "one off". 

If they want to sell it, then if they are truly a friend why not have them get an agent and sell it versus you trying to make a buck off your friend?

Am I off with my math here?

BUT, if you build another structure, all bets are off. 

Would I be better off waiting till they come around again? 

( I'm also getting advised to put them in land trusts on top of the LLC's now $$$ to avoid issues with my loan companies & having them in LLC 'sI feel EVERYWHERE you turn you hear never have rentals in your own name  put them in an LLC In retrospect , I am wondering why I would not have been petter off to keep them ALL in my personal name with 1 giant 5 million dollar umbrella policy for liability  ........

So would i structure how long and how often like " property generates X per month, monthly payment to be 50% of X for however long it takes to pay it off (would calculate)?

Here are a few resources that can help-ABCs of real estate, The Best Ever Podcast With Joe Fairless, and podcasts such as The Multifamily Take-Off, Multifamily Mavericks, The Michael Blank Podcast, and The Real Estate Monopoly Podcast (this is one that my brothers and I started). 

If you don’t need to live off of cash flow, it’s not really necessary to grow wealth from real estate.