Also, 18 months is too short a period of time to obtain a truly significant result.  

Although the study may increase the gain, the gain may be taxed at a much lower rate making it a beneficial investment.If a property is purchased with the intention to flip or own for a short period of time (less than 3 years), a cost segregation study may not be significant.SHOULD I GET A COST SEGREGATION STUDY BEFORE OR AFTER REHABBING MY PROPERTY?

Note: Short and succinct updates are provided on the https://fincen.gov/boi website but I like to get more context.

If it gets rented the owner will update the listing with a new available date; if this moves by 30 days or more you can reasonably assume it was rented.

Increase rent every single year at the renewal period.

Once renewal period comes up (if they've been paying on time and causing no trouble) you can decide if you want to keep them or move on. 

New roof, exterior painting, new flooring, bathroom updates, finished new living space What was the outcome?

They focused on renovations that appeal universally.. think updated kitchens and bathrooms - and kept meticulous records of their expenses.

If you go with a conventional loan for the cash out refinance, seasoning periods are longer which draws out the process. 

You are right about the risk that comes with the holding period but in my opinion sometimes you have to take the risk and make sure the profit that you generate is higher than the costs that you paid by holding the deal for a specific amount of time.