As active realtor, and investor I get a lot of people approaching me who want to invest. 

Interesting approach, how would multiple refi affect my credit score?

My best that I expect beat virtually all non commercial residential is over $20k/month- incredible rent growth.  

You are prepared to walk away from a deal.Not going to give you the answer (maybe if you were Army or Air Force) LOL, but here is the approach. 

Cost segregation utilizes the MACRS approach.

Can try to reposition to Class B, but neighborhood may impede these efforts.Vacancy Est: Historically 10%, but 15-20% should be used to also cover tenant nonpayment, eviction costs & damages.Tenant Pool: majority will have FICO scores of 560-620 (approaching 22% probability of default), many blemishes, but should have no evictions in last 2 years.

Although I agree with @Nathan Gesner in part, I think he is off base in categorizing a relationship based approach to landlording as "amateur".

This brings me to now, I've taken this approach and have used my own money in lending, but am also bringing in friends and family to take part in these transactions so they can earn way better returns than with CD's.

Any tools, APIs, or approaches you’d recommend?

Depending on the costs and a build to rent approach could work if you'd be willing to hold the properties as rentals.