First, the fact that this is usually written as one word marks it as an in-group term, with all the aforementioned problems that entails.
Second, even as two words, it isn’t doing what you think it’s doing. You say “trans activist” like it’s a bad thing. But would observers feel like “minority rights activist” is a bad thing? How about “women’s rights activist?” Here’s one of those cases where plain language would convey a whole lot more. How about “a certain strain of activists, who have appeared on the scene in only the last decade or so, and are arguing for radical redefinitions of commonly understood words”? It’s a mouthful, but what else are you doing? Making the problem clear is your job as a feminist activist. In successive sentences you can say something like “these activists.”