I've had squatters ( Usually people who got scammed thinking they were talking to  the "owners").

You still have to file on them and evict as usual here which can take some time. 

I get leads from other sites, but honestly, for whatever reason, they usually have lower quality leads.

Med students are usually pretty responsible, and you know they can pay with their student loans.

I usually have to reach out to someone that has it every year which is getting very old.

How do you find BRRRR properties, as they're not usually listed on Zillow and other such sites?

Usually the programs are only available on one unit properties such as single family residences, condos, planned unit development (PUD) and townhouses. 

Usually the most accurate ARV you can get is through a property appraiser. 

As far as I am aware you can rent by the room in phoenix, I am not aware of any unrelated persons ordinances but you can check with your municipality they are usually starting at > 3 people that is what I believe would limit rent by the room.

It sounds like what you want is Zillow's zestimate, which is usually in the ballpark but for some of those reasons above not entirely reliable and they have most of that data already brought in for them to use.