k3s is basically a stripped down version of Kubernetes where legacy and alpha features are removed and few components replaced with lighter-weight alternative (e.g. SQLite instead of etcd) where Minikube is a local deployment of full Kubernetes in a VM for development.

Is it safe to use for multiple nodes if etcd is removed? Won't the cluster become incoherent without it?

I believe that would be the case if the node that runs SQLite goes down. There's also an option to run k3s with etcd though.