This might be obvious to most, but I'm still making friends with CI4 and PSR in general.
The only thing in the documentation that I was missing to make this work for me was adding the namespace in Autoload:
'Denis303\ReCaptcha' => APPPATH . 'ThirdParty/vendor/denis303/codeigniter4-recaptcha'
Great job on this BTW! This saves a lot of work.