If you really see potential and like the overall quality of the property you may need to get creative on making sure you're covered if the units take longer to lease there. 

Are their listings detailed with good-quality photos?

Also how stable the area is and appreciation potential (crime rate, school quality, vacancy rate, population growth, job growth, etc.).

Louis are higher than the national average, which can impact tenant quality and property values, so thorough neighborhood research is essential.

I also agree always raise rent by minimum of $25 except for rarities such as 1st year of covid.However, I will turn around the statement “A tenant is unlikely to move for $20 a month.” to “it would be a shame to lose an outstanding tenant for $20/month”.  

In my acquaintance's experience, one of whom has triple digit number of loans being serviced by them they have produced quality and reliable results.FCI is another large, well known servicer used by many people in the industry.

I am curious, has your friend mentioned anything about the quality of life or benefits from an investment standpoint outside of affordability?

Seems like an outstanding opportunity for those who see the possibilities, can property evaluate the risk/reward opportunity, and can execute.Every suggestion that I had in my post I believe has large possibilities but because the San Jose ADU lot split rule is already approved, it seems like an almost cannot miss (assuming no RE crash).  

My best advice would be to to look at quality sponsors that have a track record to back their reputations.

I could also Airbnb it as an eco house as I will make sure air quality is optimal and buy organic mattresses but I'm not sure if Hagerstown would be a potential vacation destination.