You can also separate a node from a cluster without reinstalling it from scratch. But after removing the node from the cluster it will still have access to the shared storages! This must be resolved before you start removing the node from the cluster. A Proxmox VE cluster cannot share the exact same storage with another cluster, as storage locking doesn’t work over cluster boundary. Further, it may also lead to VMID conflicts.
Its suggested that you create a new storage where only the node which you want to separate has access. This can be a new export on your NFS or a new Ceph pool, to name a few examples. Its just important that the exact same storage does not gets accessed by multiple clusters. After setting this storage up move all data from the node and its VMs to it. Then you are ready to separate the node from the cluster.
systemctl stop pve-cluster corosync pmxcfs -l rm /etc/corosync/* rm /etc/pve/corosync.conf killall pmxcfs systemctl start pve-cluster