Apple still has 12 mini batteries in stock, and will for a while.

Might be worth trying to get the battery replaced at Apple.

Extra bonus: while it does cost money in theory, every time I've gotten Apple to replace the battery they end up breaking the screen, so I get a battery and phone replacement for free. 12 mini battery replacements _might_ be de facto free.

Just had my 12 mini battery replaced around 1 month ago, the screen survived but it was still definitely worth the money. It had degraded to around 70% capacity which was just barely preventing me from only charging overnight.