I understand it's a complex feature request and won't get priority until all the basic stuff are taken care of. I think value proposition for your control panel increases drastically with this inclusion as there are only two control panel in the market which does clustering in a proper way. Cluster Control for Database and ClusterCS for everything.
I think Docker Swarm can help here you guys and ofcourse you can have a different pricing for the sites under a cluster. I am yet to test Enhance and will do in the coming weeks. So I haven't checked the infra yet and not sure how you setup the database on the server - inside or outside of docker.
We use MySQL Replication to setup read replicas and InnoDB clusters to setup read write clusters. Both work fine. We use Ansible to set them up and monitor via MySQL Shell (not the default module). https://dev.mysql.com/doc/mysql-shell/8.0/en/
MySQL Shell module can take care of the operations around like copying the database across replica / master, adding/removing instances and changing primary or replica.
Files (ssl certifications, web server config files, wp files, user uploads) can be synced with a delay via something like syncthing. Shared storage mounts are heavy on performance. We use S3 for wp media library.
Waiting to hear your thoughts.