I don't think QAnon-style thinking is just a sideshow. I think it's the point of conservatism now. Not every pro-Trump conservative believes the specific QAnon theories, but they all believe that their enemies -- who now include not just all Democrats but every Republican who won't genuflect to Trump -- are unspeakably evil. You can give this an anodyne name like "negative partisanship," but it's far beyond that. Conservatives don't just believe the rest of us have bad ideas. They don't just think we're dangerously wrongheaded. They think we're the epitome of pure, satanic, civilization-threatening evil. They may not believe that we're all pedophiles, but they think we're all as evil as pedophiles. They may not think we're all directly tied to an elitist conspiracy intended to enchain and impoverish everyone on the planet who isn't part of the plot, but they think we're evil enough to do anything, including that and more.
So I'm not sure that Trumpism requires Trump at all. Trump is the center of a personality cult because he's the first politician at his level who seems to regard us as the unholy monsters that rank-and-file conservatives believe we are. Moreover, he seems to relish the idea of a holy war against us.
But if Trump were out of the picture, or if he seemed to waver in the fight, they'd still think that we're pure evil. Trump isn't the point -- the fight against us is the point. Trump is just, in their belief, the greatest fighter they've had.
So Q shirts at a Trump rally make perfect sense. They love Trump, but what they really want is the fight, and the fight is Q.