Use a home equity loan (fixed debt) instead of a HELOC (variable equity) and build an additional unit you can rent out in your own backyard.

Quote from @Nick Am: @Michael Plaks--> The SE taxes are STRICTLY NOT in addition. 

In southern CA, even garage conversion ADU additions are typically costing more than the value added.  

Are there any additional considerations?

Additionally, please share if this was a scam or not.

This sounds risky, are you being compensated by additional rent and have you checked with your insurance to ensure you are covered. 

I doubt you'll make 11% with the S&P, but it is probably still your best bet.

Additionally, many deals tend to fly under the radar, so networking within organizations like NARPM (National Association of Residential Property Managers) can help uncover opportunities.If anyone reading this is considering selling their property management company in Texas, feel free to share—I’m genuinely interested in connecting.

In addition to that, building your network is key.

You do that, it's going to be best place to start because I am betting from there you will seek how to simplify things.