What is the best tool for the release management for Kubernetes in a small startup?
GitLab is a solid good option for most of this (with the appropriate plan level - I don't think the CE one has everything).  It's a single-pane-of-glass for your code, security, CI/CD, metrics, etc.  You can create and deploy to kubernetes clusters from it.


You can have multiple environments and it'll store the secrets for you to customize them.
It will monitor your kubernetes apps and show deployment marks so you can see if they get better/worse performance wise before and after deployment.
You can do multi project pipelines and actions in parallel to chain things together and get things done fast.
It supports review apps and canary deployments.
It has rollback buttons and tracks every release you do.  It also has a good API, so I assume you could hook up prometheus + alert manager and make the API trigger a rollback in response to an alert.


Spinnaker has https://www.armory.io/ if you're willing to pay a bit (not sure how much).  But that should make it easy to use / effective.

I'm not amazingly fond of Jenkins, but it can do everything.  Its hard to maintain over time / with bigger projects and teams though.  But it can be customized to do anything.  You may want to look into https://jenkins.io/projects/blueocean/ (the blue ocean plugin) for CD if you're leaning this way - it looks fairly impressive and I've heard good things.