I am slowly figuring out what I is doing.
Note: Apparently just updating the entity's transform wasn't enough when resetting it as the rotation wouldn't get properly reset. Feel like that's probably a bug somewhere in Avian, but I will look into that at another time. Probably.
Source code: https://gitlab.shihogan.com/JHLeonard/drones/-/tree/br0002
In new build: press "R" to reset the rectangle! Great if you manage to give the rectangle any amount of horizontal velocity as it doesn't have any friction.