I'm not looking to replace me current company or sell it to start something new, rather supplement or diversify my income stream from multiple areas.

Value add syndicators like to refi or add supplemental loan after renovations in ~ 2 years which can increase those returns a bit but most want to show you numbers that are conservative (under promise / over deliver). 

I would of course still suggest you work a job, and use RE to supplement your salary, and slowly you'll have a point where your RE income will be able to cover all of your expenses or exceed your salary. 

There are also supplement loan features built into commercial financing that allow cash out once the property has been re-positioned.The most important factor is the profit per property...profit per unit is not as relevant. 

I am using a HELOC on my first duplex to help supplement me on the second property.

(I considered it good practice for screening future tenants.)If you're seeing cheap room rent in places like Oakland, Berkeley, or San Jose, that could be from A) larger houses with lots of rooms, B) older homeowners looking to supplement income, or C) homeowners looking to attract college students or startup employees. 

Any suggestions for good audiobooks to supplement my education?  

Broker-dealers have historically avoided the part of the industry currently being served by the platforms as well because it is extremely hard to make any money there with all of the regulatory burdens absent some sort of RIA co-investment type model that is supplemented by other sources of revenue.  

We are interested in buy and hold properties, SFRs up to four plexes, to supplement our current incomes and create passive income for the future.

I'm an IT professional looking to supplement my income and eventually exit the mandatory workforce.