Car settings in Garage read and set via canbus?

Is there a target ID for various car settings in Garage that can be used to read and set that data from canbus?

No, those are in different system. What kind of a use case you have in mind?

Primary uses case is the control module that I’m developing has a speedometer input on it to calculate and send the speed over canbus to RealDash, and if I could pull the wheel size from RealDash, I would be able to adjust the speedometer calculations from those variables.

This is possible, though its undocumented feature. Define a CAN frame like:

<frame id="your frame id" writeInterval="5000">
  <value name="Setting: Wheel Diameter" offset="0" length="1"></value> <!-- wheel diameter in inches -->
  <value name="Setting: Tire Diameter" offset="2" length="2"></value> <!-- overall tire diameter in mm -->
  <value name="Setting: Final Drive Ratio" offset="4" length="2" conversion="V*0.01"></value>
  <value name="Setting: Vehicle Weight" offset="6" length="2"></value> <!-- vehicle weight in kilograms -->

With that in your XML, RealDash will write those settings into CAN once every 5 seconds. Note to replace the frame id to what you want to use for that frame.