What are you using for Continuous Deployment (CD)?

We are currently working on supporting continuous deployments, but it seems that no suitable tools are available. We don't have particularly outlandish requirements, we want something that:

- Can be triggered via a webhook or from a GitHub PR/comment - Can queue up multiple releases (of the same or different services) and deploy them in series - Flexibility (works not only with Kubernetes)

So far we've looked at Tekton, Buildbot, and GoCD. It seems none of them support our use cases: GoCD and Tekton don't support queuing of releases and Buildbot requires so much customization that we might as well continue building our own solution.

What do other modern medium-sized software companies use to make continuous deployment a reality?