Photos Library Main & Backup advice on EHD

I was advised to make a new library on the same EHD upto 200gb, than this one big library. Is there any better advice ?

Just about the only advantage to making multiple smaller libraries is that they may open faster sometimes. Other than that there is no advantage.

Additionally I am also making a copy of this on another drive as a backup of the main library. But every time I drag and drop to update, every month or so, It takes very long as it copies the whole library and not just the latest.

Use an app that will do incremental back ups. This is a very good way to work. The first time you run the back up the app will make a complete copy of the Library. Thereafter it will update the back up with the changes you have made. That makes subsequent back ups much faster. Many of these apps also have scheduling capabilities: So set it up and it will do the back up automatically.

Example of such apps: Chronosync - but there are many others. Search on the App Store