I can understand why you can not do a total shapeshift rapidly becase your body could not handle it.
I am however curious as to why a slow partial external shapeshifting can not be done.
In other words grow a tail or wings or any kind of external appendage that is not directly tied to the internal body systems.
You can't just have a random tail poppin' outta nowhere. It counts as an external body part, meaning that blood will most definitely have to circulate through it. As far as I know we can't just create random veins without proper knowledge
you need to create a system or systems for each body part/organ, look at each piece of the human body.
you could recreate varios organs vains,arteries etc. depending on what you want to grow but it would probably take more than your lifespan to create even a little tail