Great to hear. The expectation is that the customizer gets invoked for each connection that is made once. If you're calling RedisClient.connect() once, then afterBootstrapInitialized(…) should be called once. If you're RedisClusterClient or call multiple times(), then you should see the method being called multiple times, once for each connection attempt.
Further down the lifecycle, Lettuce has an auto-reconnect feature. If the connection gets disconnected, then afterBoostrapInitialized(…) should be called again for reconnection purposes.