[switknee]

There never will be a 1 for 1 replacement because the two systems have different approaches. Why would you want a direct replacement when you could have something better?

GPOs are a windows thing and don't apply to other systems. The generic equivalent is configuration management, for which there are many solutions. Linux updates are much easier than windows updates, and many linux systems now use immutable and atomic updates by default, which further reduces risk.

For directory, openLDAP just does LDAP. DNS is done with Kea or Unbound.

Fundamentally the issue is a lack of familiarity. The only way to become familiar with a system is... to use it.