If you buy all cash you will have a better cash flow position. 

This is a little better for him because it eliminates the vacancy, so you leaving early is not as big of an issue.

But I have a sense that I'm not making the most of it and that I could potentially grow my portfolio if I made better use of the value in these properties.

I no longer qualified for traditional financing because I already own many rental properties and my DTI was high for the bank, so I purchased these in a business entity with a trusted private investor and was able to refi the 1st property/sell the 2nd property to pay back my investor and ultimately keep the better property for zero money down.

Even better...you can't take bonus depreciation on your newly acquired asset to help offset gain since it was acquired from a related party.There is a way to effectively transfer out the future appreciation, but your historical appreciation is stuck in the Corporation - you can't get that out without triggering that gain. 

(or better yet, both)If it looks like your probable income will cover all expenses and debt service, you probably have a winner.

Should I be trying to buy a 3 or 4 plex to start or would utilizing a good chunk of equity and looking for a much larger investment like a 15+ unit be better?

So the more diligence you are able to do the better.

So just like the youtube videos you can pay a lot of money for their training and you're probably not really that much further ahead to invest in your local market.If you tell us where you are or where you want to invest, we can perhaps point you to some better resources. 

I am a licensed REALTOR here in Indiana and am representing myself and my better half as buyers.