But, for instance, I sent them a notice of rent increase effective February 1 and needed them to confirm that they have accepted the new payment terms through our electronic payment system, and even after repeated notices, they are not responding.

It's heavily dependent on the types of properties and the effectiveness your processes and systems.If you don't have systems and processes in place, you may struggle to manage 30 on your own.

2.What are the best and most cost-effective solutions for managing this potential long-term issue?

@Hamidou Keita I would recommend if duplexes are limited if you can get a 4-5 bedroom house and either rent by room for house hack or splitting a big house into 2 and adding a kitchenette and separate entrance as a more cost effective ROI option 

Additionally, I collect a $3900 - $5900 option fee which helps lower the amount that I personally leave into the property and if there is a turn over and the tenant buyer doesn't exercise the option, and there needs to be repairs made to turn the property over, I usually recoup the cost of those repairs with the next option fee of $3900 - $5900 from the next tenant buyer. 

In Portland for example we are not allowed to increase rent more than once in 12 months, and it must be less than 10% increase in "effective rent".

It's a domino effect.   

Direct communication with the builder has been the most effective method in my experience.

Most alternative investments work just like pretty much all RE niches work when deployed effectively.   

It would be good to understand how one should take that view in order to effectively use that information to decide on how to proceed with a property.