I am struggling with how to develop a wordpress website that is already deployed in production without it being damaged during development. I did this by copying the site to a subdomain and then copying it to the production server. But now, when analytics, newsletter and forms are running on the production server, when I copy the version from the subdomain that does not have the current database, I lose all the current data from the production server.

How such development is done continuously while keeping the data up to date on WP?
Modifications are made mainly by adding new pages, new features, new styles/facelifts and front-end modifications.