The problem I am having trouble solving is will purchasing these types of assets allow us to scale.  

If you are looking to expand to other properties in the future, that option of a cash-out refinance or HELOC on your appreciating asset is always there. 

I suggest you look for clarity on what resources you can use or companies can assist in using a small piece of that equity as a downpayment on another high-quality asset like the one you already own.

There is a variable missing in your post that is the key to answering the question of which asset to leverage to purchase the asset you have in mind.

If she won't buy you out, then how does she think the assets should be divided when the house does sell? 

I also recommend you discuss this with your tax advisor.There are many other alternative asset classes to invest SDIRA funds into. 

Newbie here- I haven't identified any specific assets yet but I'm looking at 4plex or smaller.

I've been a RE developer for nearly 20 years working in every asset class. 

Our primary asset is a house we bought in 2021 for $975 (3% interest rate).

Real estate is a great asset but its not the only way to invest your money and not always the best way depending on your goal and commitment to it.