Hello Guido,
i really like your plugin, however the CAPTCHA is really easy to defeat, if someone wants to spam your forum. Wouldnt it be smarter to include a real CAPTCHA solution that can be configured via the wordpress options? E.g. an option for a reCAPTCHA API key, that can be set via WP-CLI?
This would really boost your plugin a lot regarding secure use!
Best regards!