There are other points to consider in order to get Germany's EU policy: Germany is rather following continuity and long term goals which are shared by most parties. AfD and to some extend Linke are exceptions.
2) Ratification processes may need Bundesrat approval which means three or four parties need to agree and not just CDU/CSU and SPD.
3) Most parties are pro-EU & at best mildly Eurosceptic (e.g. some parts of FDP).
1) EU's constitution must not be harmed regardless the costs (no exception on FoM).
2) EU membership must be worth it (no soft line on Irish border, Irish interest have primacy as long as No 1 is not harmed).