CA is an expensive market and regarding tenant laws in my eyes more complicated/difficult as a rookie landlord.

I understand I cannot legally require them to respond, unless the notice specifically requires it, like for instance, a notice of entry, but it's making it difficult to manage the property, and I'm not sure how to proceed.

They are very difficult and expensive to move. 

What you want is an agent with experience who knows how to negotiate, watch your back, navigate difficult situations, understand contracts and law, provides an objective cross-check, etc.

Duplexes are cheaper to rent and expenses are becoming difficult for the average tenant.  

As mentioned 90 day limit, which makes it difficult, and wuite pricy to get the STR certificate from the city. 

I do not know him, but the path he is leading you is a difficult path    Note difficult does not imply impossible; there are people who are successful with S8 housing    In my area s8 is so undesirable that they write laws trying to make it challenging to not allow S8 tenants.  

It's difficult to see how a tenant will be after screened and signing a lease.

. - cash flow is difficult now with long term rentals even if you pick markets that have lower purchase prices than Boston.

Add the difficult exit and your investor pool is very limited.