i take a very blunt approach to this, there isn't any medical backing for therianthropy so it doesn't make sense to gatekeep it. it's a system of belief. maybe that sounds invalidating, but i think your beliefs are real.
I recently read Unmasking Autism by Devon Price and it made me realize how important it is to be fully inclusive of your psyche in your identity.
I AM autistic. I HAVE autism. I HAVE scars, physical and mental. All of these things tell a story. Every one of these things has an impact on who I fundamentally am.
I don’t think my therianthropy and mental illness can be separated. I feel so fundamentally different from my peers that I see myself as not human. Connecting with humans is very hard for me. My anxiety causes me to want to hide and growl like a frightened animal. My hallucinations cause me to feel things that aren’t really there. Sensory issues cause sensitive hearing. My dissociation and body issues cause me to feel separate from my body. I feel that this vessel I pilot is not me, but just the tool I use to interact with the world. I am mentally ill, causing therianthropy, but I am still a therian. The cause of my therianthropy doesn’t invalidate it. I have a screwed up creature brain teehee!