Question about Screen to World Point

I'm trying to do what should be fairly simple - have a 3D GameObject (a sphere planet) be drawn in the middle of a UI rectangle (a planet info column). I pass the anchoredPosition of the RectTransform to the ScreenToWorldPoint function, but it never seems to draw the object where it needs to be. I have also tried using the position of the RectTransform and converting it to a screen coordinate using WorldToScreenPoint, but that doesn't work either. I am using the main Camera (and yes, my Z coordinate is not 0). My Camera is shooting straight on the Z axis, so there is no angle. There are 2 considerations that may be janking this process up:

1) My Camera is at a Z-2000 coordinate (for zooming in, like a dolly to a star system), and the planet should be drawn about 120 Z-units from the camera plane; so this may be causing issues

2) My System Canvas (the Canvas where the planet boxes are drawn and where the planet position would be derived) is a Screen Space-Camera, not sure if that would cause an issue.

Here's the relevant code:

 // first I get a list of Vector3 positions for each UI box and send it to a UIManager to allow the drawing function to use. This works fine
 
 foreach (GameObject gO in systemObjectsDrawnList)
 {              
        Vector3 position = gO.transform.GetComponent<RectTransform>().anchoredPosition;
        uiManagerRef.PlanetLocationsSystemView.Add(position);                   
 }
 
 // then I draw each planet. Here's where it gets janky
 
 if (planetsToDraw[x] != null)
 {
        // planet draw routine
        PlanetData pData = new PlanetData();
        Vector3 uiPoint = new Vector3(uiManagerRef.PlanetLocationsSystemView[totalPlanets].x, uiManagerRef.PlanetLocationsSystemView[totalPlanets].y, -120f);
                     
         Vector3 planetPoint = mainCamera.ScreenToWorldPoint(new Vector3(uiPoint.x, uiPoint.y, mainCamera.transform.position.z));
  
         Vector3 position = new Vector3(planetPoint.x, planetPoint.y, -120f); // set the planet position using the planetPoint coordinates from above and the y-value
                  
         GameObject planet = Instantiate(planetsToDraw[x], position, Quaternion.Euler(0, 180, 0)) as GameObject; // draw the planet
 }

But... this is not working for some reason the way I want. Any thoughts? I'm using 2020.1.17 BTW. Thanks!