>>326463 (OP)
If watching plenty of body horror films has taught me anything, it's that a gradual process would psychologically break women. The best way to work around that would be similar to how Invasion of The Bodysnatchers went: cocoons were weaved around the victims once they reached a certain stage in the infection, arising as near-identical bodywise but psychologically hivemind. The opposite would happen to monstergirls, where their bodies would be radically changed but their psyche would be effectively intact.
They would remain effectively comatose for the duration of the transformation, but the amount of energy required to grow new bones, expand muscles, and so forth would be enormous in fact nearly impossible for certain species like Arachne or Lamia. They would need some external energy source, similar to how the yolk of an egg sustains the creature within.
An equally terrifying idea would be 'nymph' monstergirls, women going through increasing periods of growth and behavioral changes to accommodate the shift. Their search for energy to sustain the transformation would be instinctual imperative bordering on madness, especially for the larger ones. It could even manifest in extreme ego death in both the nymph and chrysalis interpretations, the memories of the women turned monster seeming almost like a dream or distant thought of a past life, with very little connection to who and what they are now. Disconcerting.
I personally feel that transformation is a terrifying prospect in fiction, one that never has happy endings or good tidings. Langelaan's 'The Fly', Kafka's 'Metamorphosis', King's 'Thinner'…horror and abomination follow the genre. Monstergirls would be better conceived as a separate variant of human, rather than a terrifying, forced transformation.