To make sure that you can test CI locally, the best way I've found so far is to make sure the checks can run with Nix, and then keep the CI config itself as simple as possible and just call Nix.

As for reducing boilerplate in the CI configs, GitHub Actions is a programming language with support for functions! It's just that function calls can only appear in very limited places in the program (only inside `steps`), and to define a function, you have to create a Git repository. The function call syntax is also a bit unusual, it's written with the `uses` keyword. So there is a lot of boilerplate that you can't remove this way, though there are several other yaml eDSLs hidden in GitHub Actions that address some points of it. E.g. you can create loops with `matrix`, but again, not general-purpose loops, they can only appear in a very specific syntactic location.

To really duplicate stuff, rather than copy-pasting blocks of yaml, without using a mix of these special yaml eDSLs, in the past I've used Nix and Python to generate json. Now I'm using RCL for this (https://rcl-lang.org). All of them are general-purpose yaml deduplicators, where you can put loops or function calls anywhere you want.