The fact that Wagner's operas use German myth doesn't make them nationalistic, any more than Britten's Peter Grimes or Philip Glass's 10,000 Airplanes on the Roof are (both use stories local to where the composers lived).

In fact the only one of his operas with a storyline out of conservative ideology is his first big success, Rienzi, based on a novel set in Italy by an Englishman and first promoted by a French Jew. (Hitler seems to have liked it, for reasons which nobody can make sense of).

There certainly were nationalistic operas in the 19th century - many of Verdi's and most of Smetana's. Verdi became a hero to the Italian Fascists. Wagner wasn't in the same line of business. You could argue that Verdi had it coming because he was so explicitly political, but not Wagner.