[CI3] Integrating Altcha

Google is moving recaptcha again requiring changes so I figured what better time to look for alternatives that ideally can be run locally. It seems Altcha fits that quite well, but there exists not a lot of information on how to integrate it beyond throwing composer or something at the problem. The captcha part itself doesn't seem that complex to implement, but the validation side has me rather confused.
https://github.com/altcha-org/altcha-lib-php
https://github.com/altcha-org/altcha-starter-php

Has anyone used this before?
Am I wasting my time here when a solution for self-hosted captcha already exists?

After scratching my head for a while I think I have gotten somewhere.

Seemingly all that is needed for altcha to work is including the javascript. Then you setup the widget and it'll give you a hidden input field with a key that you can verify against the server backend.

Unfortunately it's a form submission so it's a bit annoying to actually do the verification, but other than that it should work. What strikes me is that this certainly seems much simpler than recaptcha, but without a ready-made library to include a bit of a pain to integrate.

The actual verification backend is just plain php, but it would probably work as a library as well, so the entire thing could be directly run inside CI making captcha setup really simple.