For spirit possession-based TFs I usually prefer the transformation to unfold over several days, with dreams between the main character and the spirit to illustrate either the spirit's subjugation of the MC or the MC's surrender to the spirit, or a merger between the two. I like this structure because it gives space for the MC to ignore/deny/hide/despair/accept the physical and behavioral changes when awake and externalizes the inner conflict during sleep.
Edit: While it doesn't necessarily need to go for the same length or overall tone, Burntwitch and Sizzkun's
Girl In My Dream comic is an example.