We have a few rental units and also flip houses.

We explained to her she could find someone to sub-lease with our approval but one way or another she was on the hook for rental payments through the end of her lease and she seemed unaffected by potential legal action if a suitable sublease was not found.Anyone have any advice on this situation?  

As you know, for the conventional loans from Fannie and Freddie, you can only get them if you borrow the loan yourself when you buy a single house or 2-4 units for rental.

I bought my first rental at 26!

We knew at the price the house would cash flow, then the house market went crazy and rental prices went up tremendously and the appreciated sky rocketed so this is going to be an amazing first property for our portfolio.

Sell current rental?

You can get a non-recourse loan and leverage your funds to buy 2 rentals instead of 1. 

At a certain point you do have to replace/upgrade, which is not cheap.In our experience service warranties are almost worthless, especially on a rental. 

There can be shared laundry onsite, shared parking, shared (yet separated) yard space etcAs for the math portion, I run my rental comps on rentometer and compare it to other ADU's or 1/1 units and their rent amount.

I'm looking to shift my focus from the long game of rental appreciation to the BRRR method.