But I quickly realized that scaling one house at a time wasn’t the most efficient path to financial freedom.

Most sellers of apartments will want to do a 1031 so they do not seller finance.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

I’m trying to move away from it, but was wondering if other people knew how to do it

To maximize my cashflow and gain landlording experience, I’d do STR and MTR in the other units, as well as the other bedrooms in my unit.

Based on your question, you have two core decisions to make, and they may conflict with one another - where do you want to live, and where do you want to invest? 

It will impact underwriting if you are doing conventional loans.

In this case it might be more of a 20/65/15 to get what you need out of selling the 1st. 

Don't disqualify yourself without talking to an expert. 

Never built in Houston before so take my feedback with a grain of salt but I'd put money on it being impossible to build a duplex from scratch for $125 a foot (including everything from drawings, engineering, permit fees, general conditions, labor, material, landscaping).Out here in AZ the builder cost is going to be $200 a foot at best, but add 20% on top of that for the client cost.