As far as I remember since I assisted him in many of his Discourse-related work, I remember him using a Rails script post-importation. He selected the topics by keywords in their titles, then used officially documented commands to move them, such as Administrative Bulk Operations.
I also remember that moving topics having tags, official commands, and rake tasks did not fully update some tables, nor did the related periodic Sidekiq job.
I don’t know if it’s still the case, but that might be something to pay attention to at Bulk tagged topics, then moved topics into another category, but the category tag selector doesn't show tags - #3 by Canapin.
I hope that helps!