I think the captchas should only work after 3 consecutive right answers (or something like that). Otherwise it could be a matter of brute forcing an incomplete algorithm until one works. I have a partial solution to Captcha 4 that works like that, 1 in each 10 times i find a match and could solve it (i’m still missing the mapping between the images and the text to send).
Or at least having 2 levels for each captcha, 1 for someone that manages to solve at least 1 captcha and another for someone to prove that they can repeat with confidence the captcha.