After looking at the Nebraska SOS website it looks like there might be a charge to see these documents for each company, so you might have to scrap this method. 

((Backstory))My marketing method so far has been through mail.

@Wendy Vaidic  I have heard of that method and have heard good things about that as well.

How far away might I be from reaching this point with real estate investing, using more passive methods which currently appeal to me, and thus, being able to replace my income and essentially switch jobs?

I have never had an issue with others staying in the property past the move out by using this method.

In addition to the strategies mentioned above there is also another method, which as an out of state investor I have come to favor for my buy and hold strategy in the last year and half.  

If you are worried about appreciation, chose a method of investing that injects appreciation with rehab, or if you are worried about appreciation dropping and rents are holding steady and you have cash flow then you are good.  

Chad Carson has a method he calls the Debt Snowball Plan.

Some operators are doing advertising the BRRRR method, while many offer turnkey products to investors.

But as you can see, it will take countless hours of research to figure out your exact method.