I would try first seller financing, then a bank loan, and then cash.

He found a home that he loves in Texas but can’t get approved for a VA loan because he only has his pension as income. 

Is it a conventional loan or something else?  

I would like to continue this way, with a maximum of 10 loans taken at any given time.

I've been in mortgages since 2019, and since I only do DSCR loans, I don't bother marketing to California because cash flow is non-existent.$800k for a duplex is already ridiculous, but the cherry on top is $4,100 rent.

I will be in the same position with our latest acquisition, a building I bought with a commercial loan, in about three and a half years.Your intentions are a bit muddled in one place: you want to "sever the house" to make it a duplex and then "Airbnb or rent basement."

I would look Into creative financing or DSCR loan.

We leveraged the land and got a bank loan for $1.6 Million.

One of the most powerful ways to start building a rental portfolio is by moving and using your previous primary residence as a rental, this is because you will have a lower interest rate on the mortgage than investor loans written at the same time on the same property.I regret the many times we sold our primary instead of renting it out over the past 30 years.

Otherwise you are opening the door to pay transfer taxes and likely originate a new loan.