Isn't regulatory harmonization (one of) the points of the EU?

That won’t be an issue once the law has been adopted by the EU parliament.

That's not how the EU works. If whatever is passed is an EU regulation, it will be immediately harmonized across all member states the date it goes into effect, as the regulation is law by itself. If it is a directive, each member state will have to pass their own law that follows what is laid out in that directive, but it is not optional, so there is no convincing involved.

The only convincing that needs to happen is for all the EU Council members (head of government of each member state) to vote for the regulation/directive, and the majority of the EU parliament members to vote for it.