Is there an opportunity to raise rents at lease end or to improve the units at turnover in return for higher rent? 

I believe it improves the comfortability of the tenants who lives there.But it has to make Common Sense, Dollars Sense and Business Sense.  

@Joe Vahedi Every project I've done with recently added units has had some level of difficulty with appraisals - I have had to go out of my way to prove that the additional unit is permitted and compliant with zoning in order to qualify it with the underwriter and the income from it.Here's Fannie Mae's guidance on it: https://selling-guide.fanniemae.com/Selling-Guide/Origination-thru-Closing/Subpart-B4-Underwriting-Property/Chapter-B4-1-Appraisal-Requirements/Section-B4-1-3-Appraisal-Report-Assessment/1736875861/B4-1-3-05-Improvements-Section-of-the-Appraisal-Report-10-24-2016.htm#Accessory.20UnitsLenders and underwriters can speak to the details better than I can, but I would be VERY cautious about assuming a building with an unpermitted unit (even if typical for the neighborhood) would appraise with that unit included. 

Before listing the home, the agent shouldn't have let you price it where it is, or should have suggested ways to improve the renovation you completed.

I am open to any opinions anyone can share or creative strategies I can use to improve this deal.

Due to improvements to surrounding properties and lack of supply in the local market I'm expecting 875+ per unit (Neighboring sister units just leased for 850)Lessons learned?

I also now own 4 rental properties and want to make sure I'm taking advantage of all the tax benefits possible, as I've had significant travel and expenses related to improving a fixer-upper. 

This will give you a delta between purchase price and sale price or refi after improvement costs. 

The area is definitely improving, but Zillow shows lots of rentals still on the market.In fact most other areas that I’ve looked at in this same price range seem to have the same problem - lots of vacant rentals...

A modified BRRRR, to me the word rehab has always meant a great deal of fixing and improving.