Is the DTV visa suitable for managing a condo rental income without a formal employment contract?

You are a property manager working online as a digital nomad. Write a letter explaining your work and include supporting documents like property ownership, lease agreements, rent receipts, etc.

Do you operate under any sort of business structure? Sole trader/proprietor, perhaps?

They're not interested in your income, but under the workcation category, you do need to demonstrate that you're 'doing something useful' outside of Thailand.

Under the umbrella of a registered business, it's probably easier to 'explain', as opposed to just owning income-bearing assets.

If you search, I think you'll find others in a similar situation, including those that 'manage' their investment portfolio remotely.