Or just use composite actions, it's not 2020 anymore.

Templating GitHub Actions is very powerful (I've worked with such a setup) but it has its own headaches and if you don't _need_ custom tooling better to not have it.

I can wish for improvements on the native setup without reaching out for the sledgehammer.

I think most of the pain with GitHub Actions goes away if you use actionlint, action-validator, prettier/yamlfmt in a single ci job to validate your setup. Can even add them as git hooks that automatically stage changes and give quick feedback when iterating.