an alternative option is have her be 18 but serve the story in exactly the same way as before - in the sense that there still won't be any lewds and they will interact in the very same way, with some minor changes to make the text/story fit
the good is you avoid steam's bullshit without changing much in terms of story and content, and maybe get some practice in (if you need it, stuff is great as is)
the bad? she might feel different if she is an adult. additionally, there will inevitably be people upset about this change, especially some who later in come on, expecting to lewd her but not getting any hentai
either way, the decision is yours, i just wanted to provide an alternative option to "pay loads of money to steam/remove her entirely/lewd her". i wish you a very nice day sir!