Our plan was to either sit on the land for a few years and sell it or eventually build on it.

In that case the interest is tax deferred but the rate you pay when you withdraw may be lower than your current rate or at least the same.

Eventually AirBNB will be so guest focused that more and more owners will leave the platform, then what will they do? 

I will say be prepared for a lot of rejection but you’ll eventually find someone however expect them to want a premium.  

Investing in buy-and-hold real estate is planting the seed that will eventually grow into an apple tree that produces apples for the rest of your life.

I suggest you do an LLC together and eventually put the duplex on that LLC

But back to Cozy.co, it does all the tenant background and credit screening, at their expense, allows them to input their account bank information and give permission for the bank I designate as the recipient account to make ACH withdraws on the agreed upon schedule per the lease.

I eventually blocked all of them (I think).

I could buy a titled property, but they are all a 5 minute drive from the beach or further away.My plan is to put about $20k into the house, stay there on some long weekends, rent it out when I'm not there, and think about what type of house I would like to eventually build, probably something like a 5 bedroom / 5 bath.   

People move eventually and tenants that stay beyond their lease can always get evicted by an owner-occupied buyer so there is usually a peaceful transition during transactions.