I highly agree with inappropriateness of YAML for GitLab CI/CD configuration. It does work for simple use cases and/or in small repositories - however, doing something slightly more complicated than a simple build/test/deploy pipeline takes an immense effort, with a range of hacks and workarounds.

Even the simplest things, like passing some data (e.g. a link) from one job to another requires quite an overhead (either using artifacts, storing it somewhere in KV or even something worse).

Moreover, .gitlab-ci.yml is one and only entry-point for the CI configuration, so everything goes into it. Yes, it does have concept of includes, but even that is quite limited and not sufficient for any reasonable workflow.