Going the property management route may help with the headaches, but it's going to take 10% hit on your bottom line. 

I would make sure your rents are in line with market.

Non-passive to achieve this level of return.Add the addition of the ADU makes the main unit rent controlled if over 15 years old.the work to add an ADU likely exceeds the effort of a brrrr that can achieve infinite return  Experienced investors are adding multiple ADUs by leveraging items like bonus density, committing units to affordable housing, etc.  

I recommend everyone always create their own IRR calculator as they can get pretty complex and oyu will want to understand all the inputs and outputs. we provide a basic one to those in our membership group but I always recommend build your own and understand not only the numbers but sensitivity that lines up with those inputs.If you are hard money lending thats simple as its just a column of dates and amounts for loan provided and monthly interest and points.

Its not hard to start , its getting to the finish line is the difficult part . 

Plumbing issues, sewer line replacements, and unexpected rehab surprises are part of the game.Break-ins and theft happen.

Quote from @Micah Huffman: I am considering relocating to Nashville for a job promotion that would be a 20% increase in pay possibly more.

The renter has given you the money to cover your borrowing costs and pay for the home's upkeep. 

If it earns interest that’s a bonus.

I like the idea of using a HELOC for value add projects where you can refi in 12 months to pay back your HELOC.