"Newgen" was my first pick but it doesn't also imply "said fan will stop being a fan the moment whatever they're a fan of stops being popular" the same way "niwaka" and "larper" do. Well, I think "larper" gets that nuance by extension of it meaning "fake fan". Straight up "fake fan" isn't slangy enough, either. I hate "larper" yet I'm glad it exists because I argue that it's the closest thing English has to "niwaka". No, actually, I'm just glad that English coined any fitting slang in time so I didn't have to translate it as "bandwagon fan". Bleh.
I'd suggest "poser". Same exact meaning as larper, but it's timeless, and the word is self explanatory.
Larper is derivative lingo, and its meaning was derived from another term which itself was an acronym, so its meaning isn't obvious -- particularly for non English natives. Plus it's ephemeral like most modern lingo; people a few years from now will probably have moved on to some other term and find larper cringe, and newer generations won't know what it's supposed to mean.
Meanwhile, a hundred years from now someone encountering "poser" won't even have to look it up, because the word literally is its own definition.