Today I published the followup on my topic "cluster ware coding for AEM", with the focus on what to change when you migrate your existing AEM application (in most cases without an explicit support for clustering) to AEM as a Cloud Service.
And the best: It doesn't hurt if you follow these recommendations while you are still on AEM 6.4/6.5. It doesn't have any drawbacks or performance impacts.
Part 1: General recommendations & scheduled jobs: https://cqdump.wordpress.com/2020/08/14/cluster-aware-coding-in-aem/
Part 2: Long running sessions and clustering: https://cqdump.wordpress.com/2020/08/24/long-running-sessions-and-clustering/