Whether ypu opt in or out of recording hours (not sure if ypu actually can opt out, courts will propably decide that soon enough), you cannot opt out of the max working hour limits put in law. One of the exceptions being upper management, and in Germany, funny enough, doctors...

The legal limit in Germany is 10 hours net, pkus whatever break time is applicable. One exception are mainly surveilling duties, e.g. monitoring security cameras, where the limit is 12 hours. One theoretical "work around" is declaring Saturdays normal working days, as they legally are. Those 10 hours are (used to be, not surw if that changed) calculated on the average of 5 days, by using Saturday as working day on which people are not working the average is calculated based on six days with one day adding 0 hours to tje total. Not sure if that was ever tried in court so.

Disclaimer: The above applies to Germany.

Fun fact: If employees violqte those limits, the line manager is liable personally. That liability includes fines, I know of one case where said fine was 10k Euro, and theoretically even jail time.

You cannot work contract your way around the law. Corporations still try to, even in the EU, but those sections can just be ignored by an employee.