Hey @Nitesh_Gupta ,
While you wait for further responses here,
I am not sure about your approach here,
Evaluating your upgrade decisions and their implications aren’t something that needs to be solved over a couple of forum ping pongs. You should talk to the support or solutions engineering team through your organizational-level contacts.
They would help you assess your current bottlenecks/troubles in the system and suggest the best approach to figure out the cluster upgrade roadmap which aligns better with your requirements.
Apart from that,
the release notes don’t reflect the magnitude of fixes or feature changes that went into these releases for the Search service. One thing is assured that upgrading to 6.6.X would be the best option you have if you need the greatest CB server software at the moment. (not only just for FTS)