Creating new table: Too many rows error

Hi. I'm trying to create a new table for use in a dynamic legend slicer and am getting the following error: "<pi>The resultset of a query to external data source has exceeded the maximums allowed size of '1000000' rows.</pi>".

I don't understand why this is happening. Shouldn't the resulting table contain only as many rows as categories I am using? Below is a screenshot showing all 24 of the fields I want in my LegendCategories table. Text highlighted in yellow are table names in the data model and items outlined in red are the field/column names.

And here is the DAX for the table with error message:

For ease of reading, here is the DAX code itself:

LegendCategories = 
UNION(
    SELECTCOLUMNS(
        PatientDim,
        "Category", PatientDim[RaceCategory1],
        "Field", "Race Category"
    ),
    SELECTCOLUMNS(
        PatientDim,
        "Category", PatientDim[RaceCategory2],
        "Field", "Race"
    ),
    SELECTCOLUMNS(
        PatientDim,
        "Category", PatientDim[Sex],
        "Field", "Sex"
    ),
    SELECTCOLUMNS(
        CovidVaccinationStatusFact,
        "Category", CovidVaccinationStatusFact[AgeGroup 1],
        "Field", "Age Group"
    ),
    SELECTCOLUMNS(
        CovidVaccinationStatusFact,
        "Category", CovidVaccinationStatusFact[CovidVaccinated],
        "Field", "Vaccination Status"
    ),
    SELECTCOLUMNS(
        CovidVaccinationStatusFact,
        "Category", CovidVaccinationStatusFact[Booster],
        "Field", "Booster Status"
    )
)

My expected output:

etc.

Am I doing something wrong or just misunderstanding how this works?