The honest answer to your question about where the line is: it's when you start writing your own health checks, restart logic, and deployment orchestration on top of compose. That's the tipping point where you're maintaining infrastructure instead of just running apps.
We built Miren for exactly this middle ground. Push to the cluster, it handles image building, TLS, and deployment on any Linux server. No Dockerfile, no compose, no Kubernetes. Autoscaling is on by default. It's not that Kubernetes is wrong, it's that there should be something between compose scripts and a full cluster.
We're actively working on ways of deploying already ready services in docker files to the cluster, which would probably be what you're looking for initially unless the cluster you're doing a lot of development on the cluster.