TL;DR: The current Mastodon-signup is only removing the confusion of users on first glance, because it either hides the server-choice altogether, or leaves them with a choice that is impossible to make at this point of their Mastodon-journey. Instead, it should introduce them to decentrality on a lower scale, with a handful of handpicked servers to choose from, such that the decision makes sense to them and shows them the merits and fun of the concept instead of scaring them away. Ideal would be to give them a sense of agency. Then, chances are higher that they consider migrating again in the future and eventually internalize it as a permanent option of the digital world.
Given how strident the author has been about her views, and that she is still a living person, earning royalties and materially contributing to anti-transgender causes, proclaiming oneself a fan of these books is as least saying that the author’s anti-transgender activism is not a dealbreaker. Making part of one’s identity an aspect of the books’ universe is even more so, to the point where there’s a correlation between people proclaiming their Hogwarts house in their bio and expressing anti-trans views. (TERF/“gender critical” activists having suddenly discovered that they were Slitherpuffs or whatever a few years ago, despite not having expressed any interest in YA fantasy novels before Rowling’s statements, is an observed phenomenon, to the point where such a statement in one’s bio is more likely than not to serve as a feeler for scouting out fellow anti-trans sympathisers.)