Adding a node in a running Production Cluster

We have scylladb multidc cluster setup with 3 nodes in dc1 for writes and 6 in dc2 for reads
Currently dc1 nodes have 16cpu with 80% utilisation
We want to add 3 more nodes in dc1 for smooth write operation in future as app traffic is growing

Please suggest correct method to add nodes in cluster with minimum downtime

Following above link of ScyllaDB node addition its not clear when to alter system_auth keyspaces , either before starting new node scylla service or after

Please suggest

Note: We have 1.8 tb of data present on each node
Scylla version : 5.4.1