I'm trying out SelfCI [1] for one of my projects and it's similar to what you were describing. My whole CI pipeline is just a shell script that runs the actual build and test commands, I can write a script in another language like python if I need more complexity and I can run it all locally at any time to debug.

[1] https://app.radicle.xyz/nodes/radicle.dpc.pw/rad%3Az2tDzYbAX...