it has to be reusable to be composable, ie. it needs some genericness, customization, etc. that usually requires logic, passing args, env vars, etc.

yes, writing programs (ie. an action) is the correct level of abstraction, but in general it just doesn't make much sense to have separate steps. (because for many cases unit testing and building artifacts can/should be done at the same time ... for example python/rust ... inside a container, download deps, build, then run tests, copy result to new layer, push)