Well, in case anyone wants to know, I figured it out. I added a slider element and set the Min property to 0, and the Max property to:
CountRows(Choices([@AccessibilityReport].'2023 Metrics'))
where AccessibilityReport is the name of my list and 2023 Metrics is the name of my choice column. Then I set the Default to
CountRows('2023Goals_ComboBox'.SelectedItems)
To achieve the color portion of my goal, I entered the following code into the ValueFill field:
If(
IsBlank('2023Goals_ComboBox'.SelectedItems),
RGBA(200, 200, 200, 100),
Switch(
true,
CountRows('2023Goals_ComboBox'.SelectedItems) < Slider1.Max, RGBA(45, 49, 121, 1),
CountRows('2023Goals_ComboBox'.SelectedItems) = 0, RGBA(200, 200, 200, 100),
CountRows('2023Goals_ComboBox'.SelectedItems) = CountRows(Choices([@AccessibilityReport].'2023 Metrics')), RGBA(31, 81, 49, 1)
)
)
Note that I ended up using blue and green as my indicator colors instead of red and green, per accessibility guidelines.
Also, so that no one would mess with it, I changed DisplayMode to DisplayMode.View.
Here's a picture of the progress bar when it's partially full (there are 7 items you can select from in the 2023 goals met combobox) :

Here's how it looks when all items are selected (I have an If function set on the 2023 goals progress text label so it updates if you've completed the goals):
