Would you say this is a valid concern? 

They also are military (we're near a base) and we don't have any concerns about payments.  

Yes, it is not a problem to wait for the lease to end before selling, but I am also concerned that if the market is slow at that time, the vacancy will be too long.

We purchased it the same way, with a conventional loan with 20% down, and it is doing well, the calendar is full, and we have an Air BnB superhost who also turns it over for us in between guests, and handles all guest correspondence and concerns, so it is truly passive.

Your concerns are not valid.

I am an experienced flipper, and I got into this deal with plenty of margin, so cashflowing out of it at the end is not a concern, but I need the liquid cash to get it there. 

I utilize a personal financial statement accompanied by a schedule of real estate owned document via an excel model.

Yes I'm much more concerned with taxes.

Our concern with this is that our current primary is across from the best school in our area on a cul-de-sac lot and we feel it will continue to grow at a high rate and don’t want to miss out on that. 

For example, I am a little concerned about some aspects of the business cycle recovery and a potential for a double-dip so I lean toward the safest part of capital stack which is debt (or low-debt equity).