Tach V’tat may not be universally commemorated but it is commemorated and considered the norm to do.
I don’t understand your point. Are you claiming more people commemorate tach v’tat than the Holocaust? I don’t think that’s true.
And “the Shabbos is not a commemoration for the crusades”. Well we certainly are saying a tefillah about it EVERY shabbos year round, so if those Rishonim that instituted that, then SURELY the Holocaust which was multiple times more horrific should perhaps more certainly have a specific tfilah every shabbos. Just like the crusades.
So you’re making a new point – that we should commemorate the Holocaust every Shabbos just as we do the Crusades. I can’t speak for anyone else, but when I say Av Harachamim, I don’t specifically have in mind those who died al kiddush Hashem during the Crusades, I have in mind all those who died al kiddush Hashem throughout the generations, including in the Holocaust.