Massive Freedom Grundgesetz obviously
Fun fact: Despite the continous existence of the Bayernpartei for a long time, Spanish hardline unionists continually point out to Article 21 to claim that Germany somehow bans secessionist parties lol
Sigh. Your Statement is founded on a inherent misunderstanding about how the Grundgesetz works. There are way to many tiny parties in Germany that there could be a trial before the constitutional court for every single one. There are a ton of parties that exist but are obviously against liberal democracy or the integrity of the federal republic, but that are not banned because they are completely irrelevant in practice. The MLPD (literal Stalinists) or the various neo-nazi parties (NPD, Dritter weg,DVU...) are an example for this. In the dissolution trial of the NPD, that had some relevance in the early 2000s but has lost it now, the Constitutional Court explicity said that the NPD was unconstitutional, but that it would not be dissolved because it has no relevance and is therefore not a threat to the constitution.
The Constitutional Court has repeatedly held that secessionist activities are unconstitutional. The reason why noone is going ahead and banning the Bayernpartei is because there is no serious secessionist movement in Germany. They are irrelevant. However if they would at some point become so powerful (like their counterparts in catalonia) that they could threaten the integrity of the federal republic, they would be banned.
As for the GG i like the parts about Human Rights and so on, but i would like it to be more secularist (like france) and less federalist, especially in terms of education. We currently have 16 different education systems with 16 different Abiture (a-levels) of different quality and difficulty. A unitary state would work better imo. but thats not exactly a popular opinion in Germany (and unconstitutional too).
Didn't the BP have regional representation until the late 60s or so? I would argue that's powerful enough to warrant a revision at the very least.