> Do you have any pointers for the "load on tab" idea? I didn't turn up any good results in DDG and LLMs were just hallucinating.

This is bash, not zsh, but I have this working in my dotfiles by just telling bash where to look for my custom on-demand completions:

https://github.com/NateEag/dotfiles/blob/6862726ad2ecaa3a30e...

I imagine something similar works for zsh.