Hi 
If you want to create new topics, I’d use https://docs.discourse.org/#tag/Topics/operation/createTopicPostPM, and set a short time limit between each request to prevent reaching the rate limit.
I think you could also use a rails script and use discourse/lib/topic_creator.rb at main · discourse/discourse · GitHub. You can have a look at the importer scripts to see how it’s used:
If you don’t have tons of topics, I think a quick script using the API would be the easiest way.