hm. a few thoughts, from someone who plays many gacha-heavy mobile games—
i don't mind gachas. i find them interesting, and kind of fun?
but the thing is, it is a form of gambling, and since this is a site aimed more at children/teenagers than adults, i don't feel like it's a great idea to have a gacha that's set up the way that's been proposed.
a monthly gacha with permanently retiring items wouldn't be a terrible thing, but with the prices how they are, it leaves a bit of a bad taste in my mouth. i would definitely suggest lowering either the amount of turnips per roll, and/or making the currency less expensive to begin with? additionally, something that you could do is have the items permanently retire from a lower priced gacha at the end of the month, and then add them to a shop where they sell for a higher price.
(then it would be possible—and less unintentionally skeevy—to maybe even mark them down to the original gacha's price after they've been out for a year, potentially fully retiring them at the end of that month.)
i don't know, there's just something about the thought of permanently retiring something on such a short-term regular basis that even a shop would feel a bit weird? so that's my main thought/suggestion, though i might come back with an alternative later on.