What you really want is to advance Jewish emancipation without advancing romantic nationalism. Jewish emancipation in Europe started with Napoleon, and in Germany was substantially achieved by the 1860s, which was too late. But note that by the 1920s, there was enough Jewish assimilation in Germany that Jews could believe the same racist things about the Poles that the Christian Germans believed; the right was still anti-Semitic, but there was already a liberalism that on the one hand still believed in nationalistic tropes but on the other hand was not anti-Semitic.
So the question is whether it's possible to advance Jewish emancipation. One argument that it's not is that it came out of the Enlightenment's propagation through Central Europe. Of note, British and Dutch Jews were already fairly well-assimilated and fully emancipated in the 18c, which does not mean there wasn't any anti-Semitism (the Dutch were enthusiastic collaborators in WW2), but does mean that it wasn't anything like what went on in France, Austria, Poland, Russia, and Germany. But trying to do that in Germany early may be harder, since Germany did lag the UK and Netherlands in industrialization. Emancipation is really about the formation of a large middle class, which requires industrialization.
The flip side is that Germany adopted nationalism early. Already in the 16c, there was a notion of Germany as a nation with its standard language; nobody spoke it, but people wrote in it. Italy became nationalist even earlier, through opposition to France and the Avignon papacy and Italian cardinals' unwillingness to back French popes (Louis XII tried with Georges d'Amboise, he just couldn't get Italian support). France, a more developed country than Germany, took until the 17c and state centralization.
So what you want in your POD, unless you want to go back to Martin Luther, is to get Germany to have a middle class earlier. Keep nationalism on schedule, just figure out how to get the Germans to a) develop a local middle class, and b) have some measure of political liberalism earlier. It doesn't have to be much earlier - 50 years might do the trick. You don't want to keep the far right from being anti-Semitic - it's enough to just have your average romantic define Germany in purely linguistic terms and not care toooooo much if you're Jewish or Catholic or Protestant, provided you speak standard German.
