>Now of course, in some Perfect World, GitHub could have a local runner with all the bells and whistles.

Not by GitHub, but isn't act supposed to be that?

https://github.com/nektos/act

Prefacing this with the fact that act is great, however, it has many shortcomings. Too often I've run into roadblocks, and when looking up the issue for it, it seems they are hard to address. Simpler workflows work fine with it, but more complex workflows will be much harder.

Don't put your logic in proprietary tooling. I have started writing all logic into mise tasks since I already manage the tool dependencies with mise. I tend to write them in a way where it can easily take advantage of GHA features such as concurrency, matrixes, etc. But beyond that, it is all running within mise tasks.

act is often mentioned as a drop-in replacement but I never got it to replicate GitHub actions environment. I didn't try it for this particular case, though.