The new cluster will be deployed with ECK and run the latest version available. The current cluster is running on Elastic Cloud in version 8.17.
OK, that rules out migrating through a stretched cluster.
What is the longest retention period of your data?
If this is reasonably short and you have a message queue in your ingest pipeline it might be an option to feed both clusters separately for a period of time until they hold the same data and then switch over without any downtime at all.