How can I increase the rate of double opt-in?

I’m currently getting approximately 5-15% double opt-in rates. I’m not familiar with « typical » rates, but this seems a bit low. Am I getting email spammers and how can I tell? I’m a bit suspicious about the recent increase in users trying to sign up. The numbers don’t lineup with the figures I’m seeing in Google Analytics which show much lower engagement.

I do have Recaptcha v2 setup and double opt-in email set up to prevent spamming as much as I can. I do know Brevo has an issue with Recaptcha v2 verification that I reported a while back that would allow some users to bypass Recaptcha v2, but I’m not sure if that’s been fixed.

I’m still on the free plan. I plan to upgrade to paid plans soon, but I want to be sure the signups I’m getting are genuine - and if they are, how do I increase the double opt-in rate. Thanks!