Talos Linux + Kubernetes, Rook (Ceph) storage, Cilium for networking, K8s cron and Tekton for cicd pipelines and misc tasks. There are Helm charts for all the software that would run on this.
Can run on three servers, more servers can always be added.
You could offer it an a managed service and the customer would have to do basically nothing but provide inputs on what version of the software they want running, maintenance windows, etc
If the customer were to own the Kubernetes parts they are all commodity components.
Talos Linux is a unique Linux distro in that it's sole purpose is to run a K8s cluster. The Talos team is accessible and they have reasonable commercial support options.