Hardest thing for me as Team leader is to teach people in my team, that task is done, when unit tests are written, pull request is created, approved and merged.

Not when their code compiles for a first time.

This can be solved by preparing a page called „definition of done” in your project documentation. Most Developers actually like when the expectations are written down. You probably need to mention that the documentation for the new code must be updated, release notes must be written, and other actions specific to your process should be executed.