My ideal time to have an appraisal complete is post renovation of the house, but BEFORE the tenant moves in.

Please let me know what you think.thank you for taking the time to read through this. 

Are there any unique considerations or red flags you look for compared to the traditional rental process?

These lenders originate their loans and hold them to maturity, like us, or they might sell them one at a time to an investor like you (?).

And using the process buy a new property or properties that you also intend to hold for investment use.The issue for you might be that you purchased this property with the primary intent of selling. 

I use a loan servicer who processes the borrowers payment and then sends to me.

Then start analyzing deals so you get comfortable with that process.

However, don't have any of those loans anymore (most paid off).The only thing I do that you don't, is use trusts, and love them and they have saved my bacon a number of times.When you said all those loans have paid off, are you utilizing other methods than Sub2 at this time?

The trade off course for STR route is more management/cleaning of guests coming and going/risk of no bookings vs not needing to deal with long term renters months at a time if that once they move in.

@Hunter Duplantis what @Gino Barbaro said is exactly what I was going to suggest.When you have vacancy is the time to experiment and apply your original thesis.I’m not trying to play Monday quarterback here but, you should be highly confident in where the rents should be well in advance of having a vacancy.That being said, after confirming competition rents on websites (I’d also recommend Craigslist and Facebook marketplace), you should be in alignment there.