I have had to make some large moves in the past to remove unpopular categories. The only thing to keep me sane is relying on a keyboard macro tool to handle some of the repetition of selecting the topics (x tabs and space). I use
https://www.keyboardmaestro.com/main/
Ultimately, would be nice to have a bulk move tool in Discourse, but understandably not a priority since it is not something used that often