I track the principal as a Liability account in Xero, and whenever I add it to a report, it always shows the current balance, rather than the change over the chosen time period (like with expense accounts).  

I've got about $525K in equity in the property, but lenders have told me that I cannot get a HELOC to access that equity for additional purchases because the duplex was financed as an investment - they have said TX law dictates that HELOC can only be used on primary residences.

It sounds like you’ve got a solid foundation with your rental property, and I love the ambition behind your plans to build a property management and maintenance company—it’s a great way to take control of your investments while creating additional opportunities.Best of luck to you! 

Can you do contracting work on properties and learn valuable skills that will help you remodel properties, flip them, or otherwise learn as much as possible about the construction trade? 

Quote from @Eric Huntermark: Additionally, as I'm thinking about conventional loan products as well.

Additionally, interest-bearing accounts may be mandated in certain jurisdictions, so it's important to check local regulations to ensure you're handling deposits correctly.

In addition, some states have different regulations tooI'm sure that someone that specializes in Insurance can give more insight into what is and isnt necessary or best for our properties.

Here's some input to help guide the decision between selling or renting the property, considering the specifics of the property and the potential additions like the ADU (Accessory Dwelling Unit):1.

When evaluating a property management company, it's important to look beyond just the management fee and consider the full scope of services and costs involved.For example, at our company, we charge a 10% monthly management fee, a $250 lease renewal fee (unless it’s a takeover tenant, which requires additional effort to establish a strong relationship - because usually people are changing pm companies for a reason), and the first month’s rent as a leasing fee.

You have been given valuable advice from multiple angles here.