I've started using jdx's usage[1] for my clis. It integrates neatly into clap, and can be used stand alone in scripts. It can generate completions, argparse, manpages, and more
I'm still on the fence if replacing the argparse blocks in my fish scripts is worth the hassle, but against things like old school optparse, it's far better
I’ve recently been building a similar tool [1] which defines a specification for CLIs, though the goals are slightly different to the tool you mention I think. I just added support for fish as it happens.
[1] https://github.com/fujiapple852/claptrap
Nice. I'll have to add that to my tool belt, although I think usage will still sit front and center haha