+1 for Swarm.
It's not for hyperscalers and it's got a limited feature set compared to k8s, but it's simple enough that you can really learn how it works and how to make it do what you want even if it's only a small part of your job.
If you just want redundant services, zero-downtime upgrades, and either manual-only scaling or very restricted autoscaling, Swarm is likely sufficient.
Main downside? Not available as a managed service, at least from major providers. Then again, if you're OK with managed services, you would probably prefer either a fully-managed PaaS (Heroku, Azure Web Apps, etc.) or a managed k8s.