In the EU, everything is negotiable except the free trade and free movement

That is not right. Every country must transpose all EU directives into national law. If they fail to do so, they have to pay fines.

Example

The delayed implementation of the EU directive on the protection of whistleblowers into national law is costing Germany dearly. On Thursday, the European Court of Justice (ECJ) ordered Germany to pay a fine of 34 million euros to the EU for breaching its obligations. The Luxembourg judges justified the move with the importance of the high level of protection required by the directive for whistleblowers who report breaches of EU law.

...

With its ruling in case C-149/23, the ECJ is complying with a complaint filed by the EU Commission in February 2023. The member states were actually obliged to take the necessary measures to legally comply with the provisions of the directive by the end of 2021. The German government was unable to notify the Commission of implementation because the black-red coalition initially failed to agree on a common line. At the time, the SPD wanted the law to also apply to infringements of German law. This was not just in areas such as financial services and tenders, product and food safety, data protection, the environment and health, which are regulated throughout the EU. CDU and CSU were against it.

https://www.heise.de/en/news/Late-whistleblower-protection-Germany-must-pay-a-fine-of-34-million-euros-10307299.html