In my case, I have residential rentals in one LLC, commercial properties in another, self storage in a third, and my real estate company operates in a fourth.

I think the last stat I saw said 30% of the buyers in 4th quarter were cash in DFW area. 

There are many neighborhoods in Houston that score in the 70s and up on AirDNA, so it is a good market, but you need to be precise about where you're looking.2 - the submarkets within houston that perform well for STR are nowhere near your price point. continue saving or choose a cheaper market. you're looking at the fourth largest city in the US, and the biggest city in the most moved-to state in the union.

If i paid off my first rental, it would bring in $2750/mn (before quarterly taxes/expenses) and then i would have a bucket of equity to tap into if i ever wanted to buy more property.

When you get your fourth SFR, trade up to a bigger deal with the equity accrued. 

Fourth, getting funds out of a 401K is a bit tricky; typically you have to quit, you might get an in-service distribution, other? 

They can build an inexpensive Manager's living quarters and live there, saving on buying a residence. 

Fourth, a perfectly good tenant with good references and track record can turn BAD - divorce, job loss, pregnancy, etc.  

That includes the mortgage, the insurance, if there is any electricity the landlord is responsible for, if the landlord is responsible for any of the water and sewer, not all of those bills might be every month, some might be quarterly or twice a year like the insurance, and a small percentage set aside each month for vacancies, maintenance, and eventual capital repairs.

It's a MIL apartment down in the lower quarter.