It isn't ideal to be running repairs while you're adding a new DC. When you add nodes with auto_bootstrap: false, they immediately join the ring but without data on them (which is by design). If repairs are running, it will try to populate the empty replicas on the new DC with data. It will work but it's an expensive way to bootstrap a node compared to nodetool rebuild.
Personally, I would temporarily postpone the repairs while you're building the new nodes. Cheers! >
