I'm assuming you probably have searched the WP plugins and may well have seen this, but here it is: http://escoladabiblia.org/wp-admin/plugin-install.php?tab=plugin-information&plugin=wp-captcha-booster&TB_iframe=true&width=772&height=709
I don't use it, just went looking for one so this one might have some problems, dunno. Just trying to be helpful.
@gytis @switchingsocial No experience, just searching. https://blog.templatetoaster.com/wordpress-captcha-plugins-2018/ and https://alternativeto.net/software/recaptcha/?license=opensource.
Mastodon node of Lithuanian software engineer Gytis Repečka.