Try setting up reCaptcha for payments. It adds Google’s “I’m not a robot” checkbox at checkout, which will block bots. Of course, that will only help for automated fraud transactions, but with the number of transactions you’ve had it’s very unlikely that they’re all being entered manually. reCaptcha support was added recently to core Stripe Payments, make sure you’re on the latest version. There are setup instructions here: https://s-plugins.com/stripe-payments-recaptcha-addon/
