That's something that could be made much easier if pivot_offset from Controls were turned into regular offset variables (just like Sprites), that way you would be able to animate the offset without messing with the anchoring.
In my game, I have to make a parent Control for each anchoring in the HUD, and make the animated panels their children. With that, I can animate their local positions so resizing the window won't mess things up as they still follow their parent Control, which is there just for anchoring.