Just remember that her body is already super thic by default. Like how much more balloon tits can she get? How much thicker can she get without becoming unappealing?I hadn't initially considered adding these elements you mentioned (bimbofication, weight playing, lips and breast sizing), but as we get closer to the endings of the slave route, it might not be a bad idea to include them.
If we are giving out opinions, I say just stick to the standard gyarufication (no ganguro shit though) since this seems to be set in Japan. Blonde, slight tan, tattoo, slut clothes, and maybe piercing. Bimbo lips are a western thing and it looks stupid as shit, especially when they go overboard.
Something like that should probably be the maximum "ruination" of her appearance. Personally, even this pic is a bit much, but it's at that edge of remaining decently erotic while signifying the ruin of the woman.