K8s has proven to be a massive endeavor everywhere I’ve seen it implemented.
I’ve often had teams of 2-6 to handle every technical aspect of a company with millions of visits. That includes feature development and support. K8 seems to need a large team just to keep it running.
I've had the opposite experience with K8S and docker in general. What was once pretty much always failing deployments, keeping software up to date incorrectly, K8s and docker in general keeps the burden of keeping a site running at an all time low in my lifetime of development. Like from teams to just 1 guy.
Devops it so that you can create new tenants with a script (in fact a script that is run by the signup codebase) that starts them out in cluster A.
Migration to cluster B would definitely be involved if you're not using a shared database - so potentially be ready to know your potential clients up front to get them on the correct cluster ahead of time.