I see a few balancing issues before this could truly be recommend to be approved.
With scaling I quite like the idea of balancing with it, but personally I would like to see a recommended list on the races and prefer'd creatures. Possibly faults with say one manags the ritual with a tiger but they suffer from aggressiveness, or become somewhat territorial when transformed. At first their unaffected but over time they feel pulls of natural instincts tugging at their souls in a way to reform to what their spirit animal is.
Example:
Rat - Become disinterested in others around them, find shiny objects distracting.
Mouse - Timid and shy, may be distracted by intriguing smells, and finds fear of larger creatures.
Dog - Overly playful, tends to become overly protective and loyal.
Wolf - Is rather carious and paranoid, find bones distracting for some odd reason.
Tiger - Territorial, aggressive. Tend to become overly protective of loved ones.
Leopard - Feels prideful and Vanity, sometimes loses interest in their original purpose or goal while shifted.
Cheetah - Challenging and aggressive, weakness to the chase.
Gy'waka - [Exception to the bird rule possibly since they are flightless] Power struck, prideful, and often longing feeling to hunt or be in the jungle. May turn on friends.
-Changes-
"The Druid can make the transformation permanent if he truly wishes to."
-This shouldn't be the case, druids could be regulated on how many times a day and how long they can be transformed.
If you allow for it to be permanent you'll merely have players joining just to become an animal for that only reason.
The reason I suggest times and limited would be, that if a Druid tries to spy on a place it's limited and stressful causing a bit more eventful experience rather then them sitting in one place for hours and hours just gathering information for someone else. Telling your body to change even with magic is difficult and straining on the body since it's not original to your soul and spirit.
Update Recommendations:
"This seems way too easily powergamed. What are the obvious disadvantages as a shapeshifter?"
Adding weight to this, like smaller creatures couldn't really life obvious heavy object like a ring of weighted keys, or able to lift open barred locks. As well as tails or small digits are not able to locks either, they would probably get jammed in the locking mechanisms regardless.
-This is not based of the Lore team choices, their just suggestions from myself to add to the lore.-
-This may also be updated at a later time.-